<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                    FORM 10-Q


(Mark One)

              QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15 (d) OF
[X]                    THE SECURITIES EXCHANGE ACT OF 1934
                       FOR THE QUARTER ENDED JUNE 30, 1999
                                       OR
                 TRANSITION REPORT UNDER SECTION 13 OR 15 (d) OF
[ ]                    THE SECURITIES EXCHANGE ACT OF 1934



For the transition period from _____________ to ____________

Commission File Number:  1-10883        
                         -------



                           WABASH NATIONAL CORPORATION
             ------------------------------------------------------
             (Exact name of registrant as specified in its charter)


       Delaware                                                52-1375208
       --------                                                ----------
(State of Incorporation)                                      (IRS Employer
                                                          Identification Number)


1000 Sagamore Parkway South,
     Lafayette,  Indiana                                         47905
     -------------------                                         -----
    (Address of Principal                                     (Zip Code)
     Executive Offices)

Registrant's telephone number, including area code:  (765) 771-5300
                                                     --------------

Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15 (d) of the Securities Exchange Act of 1934
during the preceding 12 months and has been subject to such filing requirements
for the past 90 days.

                                   Yes X  No
                                      ---   ---

The number of shares of common stock outstanding at August 13, 1999 was
22,978,377.



<PAGE>   2




                           WABASH NATIONAL CORPORATION

                                      INDEX

                                    FORM 10-Q



PART I - FINANCIAL INFORMATION                                             Page
                                                                           -----


Item 1.  Financial Statements

         Condensed Consolidated Balance Sheets at
         June 30, 1999 and December 31, 1998                                  1

         Condensed Consolidated Statements of Income
         for the three and six months ended June 30, 1999 and 1998            2

         Condensed Consolidated Statements of Cash Flows
         for the six months ended June 30, 1999 and 1998                      3

         Notes to Condensed Consolidated Financial Statements                 4


Item 2.  Management's Discussion and Analysis of Financial Condition
                  and Results of Operations                                  10


Item 3.  Quantitative and Qualitative Disclosures About Market Risk          15



PART II - OTHER INFORMATION


Item 1.  Legal Proceedings                                                   16


Item 2.  Changes in Securities and Use of Proceeds                           16


Item 3.  Defaults Upon Senior Securities                                     16


Item 4.  Submission of Matters to a Vote of Security Holders                 16


Item 5.  Other Information                                                   16


Item 6.  Exhibits and Reports on Form 8-K                                    16




<PAGE>   3


                  WABASH NATIONAL CORPORATION AND SUBSIDIARIES
                     CONDENSED CONSOLIDATED BALANCE SHEETS
                             (Dollars in thousands)


<TABLE>
<CAPTION>
                                                                                June 30,   December 31,
                                                                                  1999         1998  
                                                                               ----------  ------------
                                                                              (Unaudited)    (Note 1)
           
<S>                                                                            <C>          <C>
                                               ASSETS
                                               ------
CURRENT ASSETS:
     Cash and cash equivalents                                                 $  37,261    $  67,122
     Accounts receivable, net                                                    121,288       92,872
     Current portion of finance contracts                                          7,220        7,603
     Inventories                                                                 264,472      225,385
     Prepaid expenses and other                                                   14,779       19,833
                                                                               ---------    ---------
              Total current assets                                               445,020      412,815
                                                                               ---------    ---------

PROPERTY, PLANT AND EQUIPMENT, net                                               157,882      136,001
                                                                               ---------    ---------

EQUIPMENT LEASED TO OTHERS, net                                                   34,029       43,066
                                                                               ---------    ---------

FINANCE CONTRACTS, net of current portion                                         63,501       66,369
                                                                               ---------    ---------

INTANGIBLE ASSETS, net                                                            31,961       32,637
                                                                               ---------    ---------

OTHER ASSETS                                                                      17,255       13,598
                                                                               ---------    ---------
                                                                               $ 749,648    $ 704,486
                                                                               =========    =========

                                LIABILITIES AND STOCKHOLDERS' EQUITY
                                ------------------------------------

CURRENT LIABILITIES:
     Current maturities of long-term debt                                      $   2,729    $   3,089
     Accounts payable                                                            131,204      108,963
     Accrued liabilities                                                          42,028       29,507
                                                                               ---------    ---------
              Total current liabilities                                          175,961      141,559
                                                                               ---------    ---------

LONG-TERM DEBT, net of current maturities                                        163,041      165,215
                                                                               ---------    ---------

DEFERRED INCOME TAXES                                                             30,613       31,849
                                                                               ---------    ---------

OTHER NONCURRENT LIABILITIES AND CONTINGENCIES                                    20,249       20,087
                                                                               ---------    ---------

STOCKHOLDERS' EQUITY:
     Preferred stock, aggregate liquidation value of $30,600                           5            5
     Common stock, 22,978,377 and 22,965,090 shares issued and outstanding           230          230
     Additional paid-in capital                                                  236,291      236,127
     Retained earnings                                                           124,537      110,693
     Treasury stock at cost, 59,600 common shares                                 (1,279)      (1,279)
                                                                               ---------    ---------
              Total stockholders' equity                                         359,784      345,776
                                                                               ---------    ---------
                                                                               $ 749,648    $ 704,486
                                                                               =========    =========
</TABLE>


           See Notes to Condensed Consolidated Financial Statements.


                                       1


<PAGE>   4

                                        
                  WABASH NATIONAL CORPORATION AND SUBSIDIARIES
                  CONDENSED CONSOLIDATED STATEMENTS OF INCOME
                (Dollars in thousands, except per share amounts)



<TABLE>
<CAPTION>
                                                                  Three Months               Six Months
                                                                 Ended June 30,            Ended June 30, 
                                                            ------------------------   ----------------------
                                                               1999          1998         1999         1998 
                                                            ----------    ----------   ----------   ---------
                                                                   (Unaudited)               (Unaudited)
<S>                                                         <C>           <C>          <C>          <C>
NET SALES                                                   $  380,203    $  337,733   $  721,827   $ 631,345

COST OF SALES                                                  346,104       313,695      660,502     581,419
                                                            ----------    ----------   ----------   ---------

        Gross Profit                                            34,099        24,038       61,325      49,926

GENERAL AND ADMINISTRATIVE EXPENSES                              6,763         5,727       13,944      12,030

SELLING EXPENSE                                                  4,940         3,219        9,979       6,197
                                                            ----------    ----------   ----------   ---------

        Income from operations                                  22,396        15,092       37,402      31,699

OTHER INCOME (EXPENSE):
        Interest expense                                        (2,975)       (3,527)      (5,988)     (8,176)
        Accounts receivable securitization costs                (1,320)       (1,241)      (2,750)     (1,334)
        Equity in losses of unconsolidated affiliate            (1,000)         (800)      (2,000)     (1,300)
        Other, net                                                 586           747        1,816       1,007
                                                            ----------    ----------   ----------   ---------
                                                         
        Income before income taxes                              17,687        10,271       28,480      21,896

PROVISION FOR INCOME TAXES                                       7,340         4,068       11,766       8,735
                                                            ----------    ----------   ----------   ---------

        Net Income                                          $   10,347    $    6,203   $   16,714   $  13,161

PREFERRED STOCK DIVIDENDS                                          706           264        1,149         528
                                                            ----------    ----------   ----------   ---------

NET INCOME AVAILABLE TO COMMON SHAREHOLDERS                 $    9,641    $    5,939   $   15,565   $  12,633
                                                            ==========    ==========     ========    ========

Earnings per share:
        Basic                                              $      0.42    $     0.27   $     0.68   $    0.60
                                                           ===========    ==========   ==========   =========
        Diluted                                            $      0.42    $     0.27   $     0.68   $    0.60
                                                           ===========    ==========   ==========   =========

Cash dividends per share                                   $    0.0375    $    0.035   $    0.075   $    0.07
                                                           ===========    ==========   ==========   =========
</TABLE>






           See Notes to Condensed Consolidated Financial Statements.
                                        
                                        
                                       2


<PAGE>   5
                                        
                  WABASH NATIONAL CORPORATION AND SUBSIDIARIES
                CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
                             (Dollars in thousands)


<TABLE>
<CAPTION>
                                                                                  Six Months
                                                                                 Ended June 30 
                                                                           ------------------------
                                                                             1999            1998   
                                                                           ------------------------
                                                                                 (Unaudited)
<S>                                                                        <C>            <C>
CASH FLOWS FROM OPERATING ACTIVITIES:
  Net Income                                                               $  16,714      $  13,161
  Adjustments to reconcile net income to net cash
      provided by (used in) operating activities-                                 
    Depreciation and amortization                                              9,933          8,062
    (Gain) loss on sale of property, plant and equipment                        (672)            65
    Bad debt provision                                                         1,315            395
    Deferred income taxes                                                     (3,125)         2,570
    Equity in losses of unconsolidated affiliate                               2,000          1,300
    Change in operating assets and liabilities:
      Accounts receivable                                                    (29,730)        12,607
      Inventories                                                            (39,087)       (25,000)
      Prepaid expenses and other                                               6,943          1,231
      Accounts payable and accrued liabilities                                34,733         29,137
      Other, net                                                                 379           (926)
                                                                           ---------      ---------

          Net cash (used in) provided by operating activities                   (597)        42,602
                                                                           ---------      ---------

CASH FLOWS FROM INVESTING ACTIVITIES:
    Capital expenditures                                                     (31,439)       (13,889)
    Net reduction (addition) to equipment leased to others                     3,408         (5,326)
    Net additions to finance contracts                                        (9,701)       (18,712)
    Investment in unconsolidated affiliate                                    (1,799)        (1,397)
    Proceeds from the sale of property, plant and equipment                    1,185            ---
    Proceeds from sale of leased equipment and finance contacts                9,494          9,397
    Principal payments on finance contracts                                    4,801          3,166
    Other, net                                                                   ---             24
                                                                           ---------      ---------

          Net cash used in investing activities                              (24,051)       (26,737)
                                                                           ---------      ---------

CASH FLOWS FROM FINANCING ACTIVITIES:
    Proceeds from:
      Long-term revolver                                                       9,300        140,600
      Common stock, net of expense                                               164         87,485
    Payments:
      Long-term debt                                                          (2,534)        (8,834)
      Long-term revolver                                                      (9,300)      (200,600)
      Common stock dividends                                                  (1,725)        (1,397)
      Preferred stock dividends                                               (1,118)          (528)
                                                                           ---------      ---------

          Net cash (used in) provided by financing activities                 (5,213)        16,726
                                                                           ---------      ---------

NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS                         (29,861)        32,591

CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD                              67,122         14,647
                                                                           ---------      ---------

CASH AND CASH EQUIVALENTS AT END OF PERIOD                                 $  37,261      $  47,238
                                                                           =========      =========
</TABLE>

                                        



           See Notes to Condensed Consolidated Financial Statements.
                                        
                                        
                                       3



<PAGE>   6

                  WABASH NATIONAL CORPORATION AND SUBSIDIARIES

              NOTES TO CONDENSED CONSOLIDATED FINANCIAL STATEMENTS


NOTE 1.  GENERAL

         The condensed consolidated financial statements included herein have
been prepared by Wabash National Corporation and its subsidiaries (the Company)
without audit, pursuant to the rules and regulations of the Securities and
Exchange Commission. Certain information and footnote disclosures normally
included in financial statements prepared in accordance with generally accepted
accounting principles have been condensed or omitted pursuant to such rules and
regulations; however, the Company believes that the disclosures are adequate to
make the information presented not misleading. The condensed consolidated
financial statements included herein should be read in conjunction with the
financial statements and the notes thereto included in the Company's 1998 Annual
Report on Form 10-K.

         In the opinion of the Company, the accompanying financial statements
contain all material adjustments (consisting only of normal recurring
adjustments), necessary to present fairly the consolidated financial position of
the Company at June 30, 1999 and December 31, 1998, its results of operations
for the three and six months ended June 30, 1999 and 1998 and cash flows for the
six months ended June 30, 1999 and 1998.

NOTE 2.  SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

a.   New Accounting Pronouncements

         In June 1998, the Financial Accounting Standards Board (FASB) issued
SFAS No. 133, Accounting for Derivative Instruments and Hedging Activities. This
statement requires that all derivative instruments be recorded on the balance
sheet at their fair value. As amended by SFAS No. 137, Accounting for Derivative
Instruments and Hedging Activities-Deferral of the Effective Date of FASB
Statement No. 133, this standard is effective for the Company's financial
statements beginning January 1, 2001, with early adoption permitted. Management
anticipates that, due to its limited use of derivative instruments, the adoption
of SFAS No. 133, as amended by SFAS No. 137, will not have a significant effect
on the Company's results of operations or its financial position.

         The Company adopted Statement of Position No. 98-1, Accounting for the
Costs of Computer Software Developed or Obtained for Internal Use, during the
first quarter of 1999. This pronouncement specifies the appropriate accounting
for costs incurred to develop or obtain computer software for internal use. The
new pronouncement provides guidance on which costs should be capitalized, when
and over what period such costs should be amortized and what disclosures should
be made regarding such costs. The adoption of this pronouncement did not have a
material effect on the Company's results of operations or financial position.

b.   Inventories

Inventories consisted of the following (in thousands):   June 30,   December 31,
                                                           1999         1998  
                                                         --------     --------  
                                                        (Unaudited)
                  
                  Raw material and components            $109,236     $104,174
                  Work in process                          15,599       12,159
                  Finished goods                           60,546       44,743
                  Aftermarket parts                        30,865       28,733
                  Used trailers                            48,226       35,576
                                                         --------     --------
                                                         $264,472     $225,385
                                                         ========     ========




                                       4


<PAGE>   7


c.   Reclassifications

         Certain items previously reported in specific consolidated financial
statement captions have been reclassified to conform with the 1999 presentation.


NOTE 3.  SEGMENTS

         Under the provisions of SFAS No. 131, the Company has three reportable
segments; manufacturing, retail and distribution, and leasing and finance
operations. The manufacturing segment principally produces new trailers and
sells them directly to certain customers or through independent dealers. The
manufacturing segment also produces trailers for the retail and distribution
segment. The retail and distribution segment sells new and used trailers,
aftermarket parts, and performs service repair on used trailers through the
Company's retail branch network. The leasing and finance segment provides
leasing and finance programs to the Company's customers for new and used
trailers. Reportable segment information is as follows (in thousands):


<TABLE>
<CAPTION>
                                                  Retail and      Leasing       Combined                   Consolidated
                                  Manufacturing  Distribution   and Finance     Segments     Eliminations      Totals
                                  -------------  ------------   -----------     --------     ------------      ------
<S>                                  <C>           <C>            <C>           <C>            <C>            <C>
Three Months Ended
June 30, 1999
-----------------------
(unaudited)
Revenues
    External customers               $290,610      $ 80,801       $  8,792      $380,203       $    ---       $380,203
    Intersegment sales                 24,554           153          4,998        29,705        (29,705)           ---
                                     --------      --------       --------      --------       --------       --------
Total Revenues                       $315,164      $ 80,954       $ 13,790      $409,908       $(29,705)      $380,203
                                     ========      ========       ========      ========       ========       ========

Income from Operations               $ 19,960      $  1,561       $  1,790      $ 23,311       $   (915)      $ 22,396
                          


Three Months Ended
June 30, 1998
-----------------------
(unaudited)
Revenues
    External customers               $252,471      $ 79,620       $  5,642      $337,733       $    ---       $337,733
    Intersegment sales                 12,372         1,212            641        14,225        (14,225)           ---
                                     --------      --------       --------      --------       --------       --------
Total Revenues                       $264,843      $ 80,832       $  6,283      $351,958       $(14,225)      $337,733
                                     ========      ========       ========      ========       ========       ========

Income from Operations               $ 11,813      $  2,432       $  1,412      $ 15,657       $   (565)      $ 15,092
                           


Six Months Ended
June 30, 1999
-----------------------
(unaudited)
Revenues
    External customers               $549,612      $153,331       $ 18,884      $721,827       $    ---       $721,827
    Intersegment sales                 38,713           181          7,419        46,313        (46,313)           ---
                                     --------      --------       --------      --------       --------       --------
Total Revenues                       $588,325      $153,512       $ 26,303      $768,140       $(46,313)      $721,827
                                     ========      ========       ========      ========       ========       ========

Income from Operations               $ 33,226      $  2,294       $  3,083      $ 38,603       $ (1,201)      $ 37,402
                           


Six Months Ended
June 30, 1998
-----------------------
(unaudited)
Revenues
    External customers               $472,171      $147,787       $ 11,387      $631,345       $    ---       $631,345
    Intersegment sales                 49,120         1,385            726        51,231        (51,231)           ---
                                     --------      --------       --------      --------       --------       --------
Total Revenues                       $521,291      $149,172       $ 12,113      $682,576       $(51,231)      $631,345
                                     ========      ========       ========      ========       ========       ========

Income from Operations               $ 24,060      $  5,136       $  2,821      $ 32,017       $   (318)      $ 31,699
</TABLE>
                          



                                       5


<PAGE>   8
NOTE 4.  EARNINGS PER SHARE

         A reconciliation of the numerators and denominators of the basic and
diluted EPS computations, as required by SFAS No. 128, is presented below (in
thousands except per share amounts):


                                                           Weighted
                                                           Average      Earnings
                                              Income        Shares     Per Share
                                             -----------------------------------
                                                         (Unaudited)
        Three Months Ended June 30, 1999
        --------------------------------

        Basic                                 $ 9,641       22,967       $0.42
                Options                           ---           33
                Preferred Stock                   295          823
        --------------------------------      -------    ---------
        Diluted                               $ 9,936       23,823       $0.42
        ================================      =======    =========       =====
        


        Three Months Ended June 30, 1998
        --------------------------------

        Basic                                 $ 5,939       22,036       $0.27
                Options                          ---           124
        --------------------------------      -------    ---------
        Diluted                               $ 5,939       22,160       $0.27
        ================================      =======    =========       =====


        Six Months Ended June 30, 1999
        --------------------------------

        Basic                                 $15,565       22,966       $0.68
                Options                         ---             16
        --------------------------------      -------    ---------
        Diluted                               $15,565       22,982       $0.68
        ================================      =======    =========       =====


        Six Months Ended June 30, 1998
        --------------------------------

        Basic                                 $12,633       21,002       $0.60
                Options                         ---            134
        --------------------------------      -------    ---------
        Diluted                               $12,633       21,136       $0.60
        ================================      =======    =========       =====



NOTE 5.  SUPPLEMENTAL CASH FLOW INFORMATION

                                                         Six Months
                                                        Ended June 30,  
                                                     -------------------
(In thousands)                                       1999          1998
--------------------------------------------------------------------------------
Cash paid during the period for:                        (unaudited)
   Interest, net of amounts capitalized             $6,024         $7,868
   Income taxes                                      7,252          7,553
--------------------------------------------------------------------------------


                                       6


<PAGE>   9


NOTE 6.  CONTINGENCIES

     a.  Litigation

         Various lawsuits, claims and proceedings have been or may be instituted
or asserted against the Company arising in the ordinary course of business,
including those pertaining to product liability, labor and health related
matters, successor liability and possible tax assessments. None of these claims
are expected to have a material adverse effect on the Company's financial
position or its annual results of operations.

         From January 22, 1999 through February 24, 1999, five purported class
action complaints were filed against the Company and certain of its officers in
the United States District Court for the Northern District of Indiana. The
complaints purported to be brought on behalf of a class of investors who
purchased the Company's common stock between April 20, 1998 and January 15,
1999. The complaints alleged that the Company violated Section 10(b) of the
Securities Exchange Act of 1934 and Rule 10b-5 promulgated under the Act by
disseminating false and misleading financial statements and reports respecting
the first three quarters of the Company's fiscal year 1998. The complaints
sought unspecified compensatory damages and attorney's fees, as well as other
relief. In addition, on March 23, 1999 another purported class action lawsuit
was also filed in the United States District Court for the Northern District of
Indiana, naming the Company, its directors and the underwriters of the Company's
April 1998 public offering. That complaint alleged that the Company and the
individual defendants violated Section 11 of the Securities Act of 1933, and the
Company, the individual defendants as "controlling persons" of the Company, and
the underwriters are liable under Section 12 of that Act, by making untrue
statements of material fact in and omitting material facts from the prospectus
used in that offering. The complaint sought unspecified compensatory damages and
attorney's fees, as well as other relief. Both the Securities Exchange Act
complaints and the Securities Act complaint arise out of the restatement of the
Company's financial statements for the first three quarters of 1998. At a
hearing on May 10, 1999 and in an order entered on June 22, 1999, Judge Allen
Sharp consolidated the six pending cases under the caption In re Wabash National
Corporation Securities Litigation, No. 4:99CV0003AS, and established a schedule
for further proceedings. Pursuant to the order, selected lead plaintiffs filed a
Consolidated Class Action Complaint on July 6, 1999. The consolidated complaint
repeats the claims made in the original complaints respecting the restatement
and also alleges that the loss contingency for certain excise taxes, which
Wabash disclosed on January 19, 1999, should have been recorded earlier. Under
the schedule established by the Court, defendants may file motions to dismiss
the consolidated complaint not later than September 7, 1999. Plaintiffs'
opposing briefs will be due on October 4, 1999, and any reply by defendants will
be due not later than October 11, 1999.

         The Company believes the allegations in the consolidated complaint are
without merit, and intends to defend itself and its directors and officers
vigorously. The Company believes the resolution of the lawsuit, including any
Company indemnification obligations to its officers and directors and to the
underwriters of its April 1998 public offering (as to which the Company is
self-insured), will not have a material adverse effect on its financial position
or future results of operations; however, at this early stage of the
proceedings, no assurance can be given as to the ultimate outcome of the case.


                                       7


<PAGE>   10


     b.  Environmental

         The Company generates and handles certain material, wastes and
emissions in the normal course of operations that are subject to various and
evolving Federal, state and local environmental laws and regulations.

         The Company assesses its environmental liabilities on an on-going basis
by evaluating currently available facts, existing technology, presently enacted
laws and regulations as well as experience in past treatment and remediation
efforts. Based on these evaluations, the Company estimates a lower and upper
range for the treatment and remediation efforts and recognizes a liability for
such probable costs based on the information available at the time. As of June
30, 1999, the estimated potential exposure for such costs ranges from
approximately $0.7 million to approximately $2.7 million, for which the Company
has a reserve of approximately $1.3 million. These reserves were recorded for
exposures associated with the costs of environmental remediation projects to
address soil and ground water contamination at certain of its facilities as well
as the costs of removing underground storage tanks at its branch service
locations. The possible recovery of insurance proceeds has not been considered
in the Company's estimated contingent environmental costs.

         The Company acquired two new manufacturing sites in July, 1998 in
connection with its acquisition of a trailer flooring business in Arkansas (the
"Cloud Acquisition") and voluntarily disclosed to the United States
Environmental Protection Agency (EPA) and the Arkansas Department of Pollution
Control and Ecology (ADPC&E) potential soil and groundwater contamination. In
association with both the EPA and the ADPC&E, the Company has submitted a
sampling plan to ADPC&E for monitoring and any required remediation. This matter
is at an early stage and it is not possible to predict the outcome with
certainty. The Company has recorded a reserve of $1.0 million related to these
issues based on currently available information and does not believe the outcome
of this matter will be material to the consolidated results of operations or
financial condition of the Company. The Company is indemnified by the Sellers of
the acquired companies and the Company believes that these matters would be
covered by the indemnification.

         Future information and developments will require the Company to
continually reassess the expected impact of these environmental matters.
However, the Company has evaluated its total environmental exposure based on
currently available data and believes that compliance with all applicable laws
and regulations will not have a materially adverse effect on the consolidated
financial position of the Company.

c.       Tax Assessment

         On December 24, 1998, the Company received notice from the Internal
Revenue Service that it intends to assess federal excise tax on certain used
trailers restored by the Company for a single customer during 1996 and 1997. The
Company strongly disagrees with and intends to vigorously contest the
assessment. In applying generally accepted accounting principles, the Company
recorded a $4.6 million accrual in 1998 for this loss contingency that is
reflected in the accompanying Condensed Consolidated Balance Sheets. The Company
continued the restoration program with the same customer during 1998 and 1999.
As a result, the Company has a contingent liability and a corresponding
receivable in the accompanying Condensed Consolidated Balance Sheets of $4.3
million and $2.4 million at June 30, 1999 and December 31, 1998, respectively.
The customer has agreed to indemnify the Company for any potential excise tax
assessed by the IRS for trailers sold and delivered after January 1, 1998.



                                       8


<PAGE>   11


     d.  Year 2000

         The Year 2000 issue arises because many computerized systems use two
digits rather than four to identify a year. The effects of the Year 2000 issue
may be experienced before, on, or after January 1, 2000, and, if not addressed,
the impact on operations and financial reporting may range from minor errors to
significant systems failure, which could affect an entity's ability to conduct
normal business operations. It is not possible to be certain that all aspects of
the Year 2000 issue affecting an entity, including those related to the efforts
of customers, suppliers, or other third parties, will be fully resolved.










                                       9




<PAGE>   12



I
TEM 2.  MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL 
         CONDITION AND RESULTS OF OPERATIONS

              CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

         This report and the information incorporated by reference, may include
"forward-looking statements" within the meaning of Section 27A of the Securities
Act and Section 21E of the Exchange Act. We intend the forward-looking
statements to be covered by the safe harbor provisions for forward-looking
statements in these sections. All statements regarding our expected financial
position, operating results and our business strategy are forward-looking
statements. These statements can sometimes be identified by our use of
forward-looking words such as "may," "will," "anticipate," "estimate," "expect,"
or "intend." Known and unknown risks, uncertainties and other factors could
cause the actual results to differ materially from those contemplated by the
statements. The forward-looking information is based on various factors and was
derived using numerous assumptions.

         Although we believe that our expectations that are expressed in these
forward-looking statements are reasonable, we cannot promise that our
expectations will turn out to be correct. Our actual results could be materially
different from and worse than our expectations. Important risks and factors that
could cause our actual results to be materially different from our expectations
include the factors that are listed in our Registration Statement on Form S-3
(SEC File No. 333-48589) under the heading "Risk Factors."

RESULTS OF OPERATIONS

The Company has three reportable business segments;

    *    manufacturing,
    *    retail and distribution, and
    *    leasing and finance operations

The manufacturing segment principally produces trailers and related components
and sells to customers who purchase directly from the Company or through
independent dealers. The manufacturing segment also produces trailers and
related components for the Company's retail and distribution segment. The retail
and distribution segment sells new and used trailers, aftermarket parts, and
performs service repair on used trailers through its retail branch network. The
leasing and finance segment provides leasing and finance programs to its
customers for new and used trailers.

         Net Sales

         Consolidated net sales for the three-month period ended June 30, 1999
increased approximately $42.5 million or 12.6% compared to the same period in
1998 and were $90.5 million or 14.3% higher for the six-month period ended June
30, 1999 compared to the same period in 1998. Sales increased in all of the
Company's business segments.

         The manufacturing segment's net sales rose 19.0% or $50.3 million in
the second quarter of 1999 compared to the same period in 1998 and were $67.0
million or 12.9% higher for the six-month period ended June 30, 1999 compared to
the same period in 1998. These increases were driven by a 14.6% increase (16,600
units vs. 14,480 units) and a 15.5% increase (31,780 units vs. 27,510 units) in
the number of new trailers sold during the three and six-month periods ended
June 30, 1999 compared to the same periods in 1998, respectively. Strong demand
for the Company's proprietary DuraPlate(TM) trailer continues to drive increased
new trailer revenues in the manufacturing segment.



                                       10

<PAGE>   13


         The retail and distribution segment's net sales rose 0.2% or $0.1
million in the second quarter of 1999 compared to the same period in 1998 and
were $4.3 million or 2.9% higher for the six-month period ended June 30, 1999
compared to the same period in 1998. The increase in net sales was due to an
increase in used trailer revenues, which was offset partially by decreases in
new trailer revenues, aftermarket parts revenues and service revenues. The
increase in used trailer revenues is a result of the Company's strategy to
increase its used trailer marketing efforts and capacity through its retail
branch network, particularly related to used trailers taken in trade on new
trailer transactions. Inclement weather conditions throughout the mid-western
part of the country during the first quarter of 1999 coupled with the temporary
adverse impact of the Company's consolidation of its aftermarket parts
operations negatively impacted sales of new trailers, aftermarket parts and
service compared to 1998. The number of new trailers sold in the retail and
distribution segment during the three and six-month periods ended June 30, 1999,
were approximately 1,500 and 2,400, respectively, compared to approximately
1,500 and 2,800 in the three and six-month periods ended June 30, 1998,
respectively.

         The Company continues to pursue its branch expansion strategy of
approximately 20 locations; which includes the replacement of certain existing
sites within the same market as well as the addition of locations in new markets
throughout North America. The majority of the new locations are expected to
become operational during the second half of 1999 and the first half of 2000
with the remainder becoming operational thereafter.

         The leasing and finance segment's net sales rose 119.5% or $7.5 million
in the second quarter of 1999 compared to the same period in 1998 and were
117.1% or $14.2 million higher for the six-month period ended June 30, 1999
compared to the same period in 1998. These increases were primarily due to an
increase in the sale of used trailers to external customers and to the retail
branch network, for used trailers coming off lease.

         Gross Profit

         Gross profit as a percentage of sales totaled 9.0% for the second
quarter of 1999 compared to 7.1% for the same period in 1998 and totaled 8.5%
for the six-month period ended June 30, 1999 compared to 7.9% for the same
period in 1998. The increase in gross profit percentages in 1999 is primarily
due to an improved production mix toward more proprietary products and reduced
material costs (specifically hardwood flooring costs) following the Company's
July, 1998 Cloud Acquisition. Partially offsetting these factors is the
temporary adverse impact caused by the Company's consolidation of its two
aftermarket parts operations, the continued reconfiguration of its retail and
distribution network and the severe adverse weather impact incurred during the
first quarter of 1999.

         Income from Operations

         Income from operations as a percentage of sales was 5.9% for the second
quarter of 1999 compared to 4.5% for the same period in 1998 and was 5.2% for
the six-month period ended June 30, 1999 compared to 5.0% for the same period in
1998. Income from operations in 1999 was impacted primarily by the increase in
gross profit margins previously discussed offset somewhat by increased selling,
general and administrative expenses. The increase in selling, general and
administrative expenses primarily reflects increased selling expenses incurred
in the retail and distribution segment principally to support increased used
trailer sales, as previously discussed, as well as other sales-related expenses
in the manufacturing segment associated with higher sales volume.



                                       11


<PAGE>   14


         Other Income (Expense)

         Interest expense for the second quarter of 1999 totaled $3.0 million
compared to $3.5 million for the same period in 1998 and totaled $6.0 million
for the six-month period ended June 30, 1999 compared to $8.2 million for the
same period in 1998. The decrease in interest expense primarily reflects lower
borrowings on the Company's revolving credit facility during 1999 as a result of
the cash generated from the Company's accounts receivable securitization
facility established in March 1998 and the Company's April 1998 common stock
offering.

         Other, net for the second quarter of 1999 totaled $0.6 million compared
to $0.7 million for the same period in 1998 and totaled $1.8 million for the
six-month period June 30, 1999 compared to $1.0 million for the same period in
1998. Other, net is comprised primarily of interest income and gain on the sale
of property, plant and equipment.

         Taxes

         The provision for income taxes for the three and six month periods
ended June 30, 1999 of $7.3 million and $11.8 million, respectively, represents
41.5% and 41.3% of pre-tax income for the periods compared to the provision of
$4.1 million and $8.7 million, or 39.6% and 39.9% of pre-tax income,
respectively, for the same periods in 1998. The effective tax rates are higher
than the Federal statutory rates of 35% due primarily to state income taxes.


LIQUIDITY AND CAPITAL RESOURCES

         Operating Activities

         Net cash used in operating activities was $0.6 million during the first
six months of 1999 primarily as a result of net income, the add-back of non-cash
charges for depreciation and amortization, and changes in working capital.
Changes in working capital consisted primarily of increased inventory and
accounts receivable balances due to higher sales activity offset in part by
related increases in accounts payable and accrued liabilities.

         On March 31, 1998, the Company replaced its existing $40 million
receivable sale and servicing agreement with a new three-year trade receivable
securitization facility. The new facility allows the Company to sell, without
recourse on an on-going basis, all of its accounts receivable to Wabash Funding
Corporation (Funding Corp.), a wholly-owned unconsolidated subsidiary of the
Company. Simultaneously, the Funding Corp. has sold and, subject to certain
conditions, may from time to time sell an undivided interest in those
receivables to a large financial institution. At June 30, 1998 and 1999, $83
million and $105 million of proceeds, respectively, were outstanding under this
facility.




                                       12


<PAGE>   15


         Investing Activities

         Net cash used in investing activities of $24.1 million during the six
months ended June 30, 1999 was primarily due to capital expenditures of $31.4
million partially offset by a $8.0 million net cash inflow related to the
reduction in the Company's leasing portfolio.

         Capital expenditures during the period were primarily associated with
the following:

         *  Increasing automation within the Company's manufacturing operations
            in Lafayette, Indiana;

         *  Development of a new state-of-the-art painting and coating system
            and plant expansion at its trailer manufacturing facility in
            Huntsville, Tennessee;

         *  Increasing capacity and manufacturing productivity at its hardwood
            flooring operations in Arkansas;

         *  The acquisition of property related to the Company's branch
            expansion strategy; and 

         *  The development of a new computer system in the Company's retail
            branch network


The Company anticipates future capital expenditures related to the continuation
of the capital projects previously discussed and other activities to be $40 to
$60 million over the next twelve to twenty-four months. The decrease in the
Finance Company's leasing portfolio was the result of the sale of certain used
trailers during the period which were previously leased.

         Financing Activities

         Net cash used in financing activities of $5.2 million during the six
months ended June 30, 1999 was primarily due to the paydown of long-term debt of
$2.5 million and the payment of common stock dividends and preferred stock
dividends of $2.8 million in the aggregate.

         Other sources of funds for capital expenditures, continued expansion of
businesses, dividends, principal repayments on debt, and working capital
requirements are expected to be cash from operations, additional borrowings
under the credit facilities, and term borrowings. The Company believes that
these funding sources will be adequate for its anticipated requirements.

YEAR 2000 READINESS DISCLOSURE

         The Company continues to address the impact of the Year 2000 issue on
its business. If not corrected certain computer applications may fail or create
erroneous results at the year 2000. Specifically, with respect to the Company,
this includes applications within information technology (IT) as well as non-IT
equipment and machinery that may contain embedded date-sensitive
microcontrollers or microchips.

         Information Technology Systems

         The Company's assessment of all business critical IT hardware and
software is complete and will continue throughout the remainder of 1999 on any
subsequent acquisitions of IT systems. It has been determined that many of the
Company's applications and systems are Year 2000 ready, however, it will be
necessary to modify or replace other applications and systems. During this
assessment, it was determined that systems in place within the Company's retail
and distribution network and certain of its manufacturing operations are not
Year 2000 ready. As a result, during 1999, the Company has installed and will
install new application systems or modify existing systems within these areas in
order to be Year 




                                       13


<PAGE>   16

2000 ready. The installation and testing of the Company's retail and
distribution system is scheduled for completion during the fourth quarter of
1999. Other modifications are being made to the hardware and software at certain
of the Company's manufacturing locations. These modifications are scheduled to
be completed during 1999. If such projects are not completed timely, the Year
2000 issue could create a disruption in normal business operations but would not
have a material impact on the operations of the Company. The Company's plan for
IT items includes the following phases and timeline: (1) Assessment and Strategy
- completed in 1999 and (2) Design, Implementation, Testing and Validation - in
process and scheduled to be substantially completed during the fourth quarter
1999.

         Non-Information Technology Equipment

         The Company's assessment of business critical non-IT manufacturing
equipment is complete and will continue throughout the remainder of 1999 on any
subsequent acquisitions of non-IT manufacturing equipment. It is expected that
the necessary replacements or upgrades on this equipment, resulting from this
assessment, will be completed during the remainder of 1999. If upgrades and
replacements are not completed timely, the Year 2000 issue could have a material
adverse impact on the operations of the Company. Assuming the "worst case
scenario," without the use of essential manufacturing equipment, production
would cease or be severely restricted until Year 2000 ready equipment can be
installed. Testing of these systems will continue throughout 1999 as new
manufacturing equipment is acquired.

         External Parties

         The Company has contacted its vendors and suppliers regarding the
status of their Year 2000 readiness. Many vendors have given a positive
indication that they are or will be ready. A follow-up inquiry program is in
place with the parties identified as business critical. This process is ongoing
and will continue throughout the remainder of 1999. While compliance issues may
be identified and addressed, this process may not fully ensure these parties'
Year 2000 readiness. Disruptions in the operations of these parties could have
an adverse financial and operational effect on the Company. The Company has
formulated a contingency plan in the event business critical vendors do not
achieve Year 2000 readiness and suffer substantial disruptions in their
operations. This plan includes advance purchasing of critical production
materials, identification of substitute materials already existing at the
manufacturing sites and to contact alternate vendors previously identified for
availability of critical materials or viable substitutes. In the event of IT
failure in this area, purchase orders can be handwritten and forwarded via U.S.
mail or fax.

         The Company is conducting a follow-up inquiry with its banks on
previous expectations of Year 2000 readiness by the close of the third quarter
of 1999. The Company continues identifying alternate banking relationships in
the event that its current banks are determined not to be Year 2000 ready. In
the event alternative banking relationships are required, they are expected to
be in place by the third quarter in 1999.

         Costs of Compliance

         The Company estimates the total costs to be incurred in installing new
application systems in the retail and distribution network and certain
manufacturing operations, along with costs associated with Year 2000 readiness
to be between $5.9 to $6.4 million. Through June 30, 1999, the Company has spent
approximately $4.9 million, including internal labor costs, related to such
activities.

         Management believes that, with modifications to existing software and
conversions to new software and hardware, the Year 2000 issue is not likely to
materially impact the Company's results of operations or financial position. The
Company expects all of its internal business critical IT and 



                                       14

<PAGE>   17


non-IT systems to be Year 2000 ready and therefore no contingency plan is in
place in the event of a particular system not being Year 2000 ready. Such a plan
will be developed if it becomes clear that the Company is not going to achieve
its scheduled readiness objectives. However, because most computer systems are
interdependent by nature, there can be no assurance that the systems of other
companies on which the Company's systems rely, will be timely converted and not
have an adverse effect on the Company's systems.

BACKLOG

         The Company's backlog of orders was approximately $1.2 billion and $1.0
billion at June 30, 1999 and December 31, 1998, respectively. The Company
expects to fill a majority of its backlog within the next twelve months.

NEW ACCOUNTING PRONOUNCEMENTS

         In June 1998, the Financial Accounting Standards Board (FASB) issued
SFAS No. 133, Accounting for Derivative Instruments and Hedging Activities. This
statement requires that all derivative instruments be recorded on the balance
sheet at their fair value. As amended by SFAS No. 137, Accounting for Derivative
Instruments and Hedging Activities-Deferral of the Effective Date of FASB
Statement No. 133, this standard is effective for the Company's financial
statements beginning January 1, 2001, with early adoption permitted. Management
anticipates that, due to its limited use of derivative instruments, the adoption
of SFAS No. 133, as amended by SFAS No. 137, will not have a significant effect
on the Company's results of operations or its financial position.

         The Company adopted, Statement of Position No. 98-1, Accounting for the
Costs of Computer Software Developed or Obtained for Internal Use, during the
first quarter of 1999. This pronouncement specifies the appropriate accounting
for costs incurred to develop or obtain computer software for internal use. The
new pronouncement provides guidance on which costs should be capitalized, when
and over what period such costs should be amortized and what disclosures should
be made regarding such costs. The adoption of this pronouncement did not have a
material effect on its results of operations or financial position.


I
TEM 3.  QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISKS

         The Company has limited exposure to financial risk resulting from
volatility in interest rates and foreign exchange rates. As of June 30, 1999,
the Company had approximately $6 million of LIBOR based debt outstanding under
its Revolving Credit Facility and $105 million of proceeds from its accounts
receivable securitization facility, which also requires LIBOR based interest
payments. A hypothetical 100 basis-point increase in the floating interest rate
from the current level would correspond to a $1.1 million increase in interest
expense over a one-year period. This sensitivity analysis does not account for
the change in the Company's competitive environment indirectly related to the
change in interest rates and the potential managerial action taken in response
to these changes.

         The Company enters into foreign currency forward contracts (principally
against the German Deutschemark and French Franc) to hedge the net
receivable/payable position arising from trade sales (including lease revenues)
and purchases primarily with regard to the Company's European RoadRailer
operations. The Company does not hold or issue derivative financial instruments
for speculative purposes. A hypothetical 10% adverse change in foreign currency
exchange rates would have an immaterial effect on the Company's financial
position and results of operations. Additional disclosure related to the
Company's risk management policies are discussed in Note 2 to the Consolidated
Financial Statements included in the Company's 1998 Annual Report on Form 10-K.




                                       15


<PAGE>   18

PART II - OTHER INFORMATION


ITEM 1.  LEGAL PROCEEDINGS

         See Footnote 6 to the Condensed Consolidated Financial Statements for
information related to Legal Proceedings.


ITEM 2.  CHANGES IN SECURITIES AND USE OF PROCEEDS

         Not Applicable


ITEM 3.  DEFAULTS UPON SENIOR SECURITIES

         Not Applicable


ITEM 4.  SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS

         The Company held its annual meeting of security-holders on May 6, 1999,
at which time the following nominees were elected to the Board of Directors:

                                                             WITHHOLD AUTHORITY
                NOMINEES                 FOR                      TO VOTE
                --------                 ---                      -------
         Richard E. Dessimoz          20,542,605                  373,781
         Donald J. Ehrlich            20,552,605                  363,781
         John T. Hackett              20,471,378                  445,008
         E. Hunter Harrison           20,469,480                  446,906
         Mark R. Holden               20,530,605                  385,781
         Ludvik F. Koci               20,471,480                  444,906


ITEM 5.  OTHER INFORMATION

         None


ITEM 6.  EXHIBITS AND REPORTS ON FORM 8-K

(a)      Exhibits:

         27.01    Financial Data Schedule

(b)      Reports on Form 8-K:

         The Company did not file any reports on Form 8-K during the quarter
ended June 30, 1999




                                       16


<PAGE>   19



SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized.

                                        WABASH NATIONAL CORPORATION

Date:  August 13, 1999                  By:  /s/ Rick B. Davis
       ---------------                       -----------------
                                                 Rick B. Davis
                                                 Corporate Controller
                                                 (Principal Accounting Officer)
                                                          and
                                                 Duly Authorized Officer









                                       17






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<ARTICLE> 5
<LEGEND>
THIS SCHEDULE CONTAINS SUMMARY FINANCIAL INFORMATION EXTRACTED FROM THE
CONSOLIDATED BALANCE SHEET, STATEMENT OF INCOME, AND STATEMENT OF CASH FLOW AND 
IS QUALIFIED IN ITS ENTIRETY BY REFERENCE TO SUCH FINANCIAL STATEMENTS.
</LEGEND>
<MULTIPLIER> 1
<CURRENCY> U.S.
       
<S>                             <C>
<PERIOD-TYPE>                   6-MOS
<FISCAL-YEAR-END>                          DEC-31-1999
<PERIOD-START>                             JAN-01-1999
<PERIOD-END>                               JUN-30-1999
<EXCHANGE-RATE>                                      1
<CASH>                                          37,261
<SECURITIES>                                         0
<RECEIVABLES>                                  121,288
<ALLOWANCES>                                         0
<INVENTORY>                                    264,472
<CURRENT-ASSETS>                               445,020
<PP&E>                                         157,882
<DEPRECIATION>                                       0
<TOTAL-ASSETS>                                 749,648
<CURRENT-LIABILITIES>                          175,961
<BONDS>                                        163,041
<PREFERRED-MANDATORY>                                0
<PREFERRED>                                          5
<COMMON>                                           230
<OTHER-SE>                                     359,549
<TOTAL-LIABILITY-AND-EQUITY>                   749,648
<SALES>                                        721,827
<TOTAL-REVENUES>                               721,827
<CGS>                                          660,502
<TOTAL-COSTS>                                  660,502
<OTHER-EXPENSES>                                23,923
<LOSS-PROVISION>                                     0
<INTEREST-EXPENSE>                               5,988
<INCOME-PRETAX>                                 28,480
<INCOME-TAX>                                    11,766
<INCOME-CONTINUING>                             16,714
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<NET-INCOME>                                    16,714
<EPS-BASIC>                                       0.68
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