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

                                    FORM 10-Q/A
                                 (AMENDMENT NO. 1)

X     Quarterly report pursuant to Section 13 or 15(d) of the Securities 
      Exchange Act of 1934 for the quarterly period ended June 30, 1998 or

      Transition report pursuant to Section 13 or 15(d) of the Securities 
      Exchange Act of 1934 for the transition period from _________to_________

Commission file number:  000-20923

                                 SUMMIT DESIGN, INC.
             (Exact name of registrant as specified in its charter)


              DELAWARE                                   93-1137888 
   (State or other jurisdiction of                    (I.R.S. Employer 
   incorporation or organization)                   Identification Number)

                              9305 S. W. GEMINI DRIVE,
                              BEAVERTON, OREGON  97008
                      (Address of principal executive office)
        Registrant's Telephone number, including area code:  (503) 643-9281

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 (or for such shorter period that the 
registrant was required to file such reports), and (2) has been subject to 
such filing requirements for the past 90 days.   Yes   X      No        
                                                      ---         ---
As of August 11, 1998, the Registrant had outstanding 15,237,624 shares of 
Common Stock.

<PAGE>

                                SUMMIT DESIGN, INC.
                                       INDEX
                                          

PART I     FINANCIAL INFORMATION

Item 1    Consolidated Financial Statements

          Consolidated Balance Sheets as of June 30, 1998 (unaudited) 
          and December 31, 1997.                                             3
                                                                              
          Consolidated Statements of Operations for the three month 
          periods ended June 30, 1998 and 1997 and for the six month 
          periods ended June 30, 1998 and 1997 (unaudited).                  4
                                                                              
          Consolidated Statements of Cash Flows for the 
          six month periods ended June 30, 1998 and 1997 (unaudited).        5
                                                                              
          Notes to Consolidated Financial Statements.                        6

Item 2    Management's Discussion and Analysis of Financial
          Condition and Results of Operations                                9

PART II   OTHER INFORMATION

Item 1    Not Applicable

Item 2    Changes in Securities and Use of Proceeds                         28

Item 3    Not Applicable

Item 4    Submission of Matters to a Vote of Security Holders               28

Item 5    Not Applicable

Item 6    Exhibits and Reports on Form 8-K                                  29

Signature                                                                   30

Exhibit Index                                                               31

This Quarterly Report on Form 10-Q/A ("Form 10-Q/A") is being filed as 
Amendment No. 1 to the Registrant's Quarterly Report on Form 10-Q filed with 
the Securities and Exchange Commission on August 14, 1998 ("Form 10-Q") for 
the purpose of amending Item 1 in its entirety.


                                       -2-
<PAGE>

                                SUMMIT DESIGN, INC.
                            CONSOLIDATED BALANCE SHEETS
                                   (in thousands)
<TABLE>
<CAPTION>
                                                          June 30, 1998        December 31, 1997
                                                          -------------        -----------------
                                                           (Unaudited)                          
<S>                                                       <C>                  <C>              
                  ASSETS                                                                        
Current assets:                                                                                 
     Cash and cash equivalents.......................       $  24,768                $  19,973  
     Accounts receivable, net........................           5,672                    5,131  
     Prepaid expenses and other......................             323                      540  
     Deferred income taxes...........................           1,268                    1,209  
                                                          -------------        -----------------
          Total current assets.......................          32,031                   26,853  
                                                                                                
Furniture and equipment, net.........................           3,208                    2,698  
Intangibles, net.....................................           1,376                    1,622  
Deferred taxes.......................................             555                      533  
Deposits and other assets............................           1,649                    1,055  
                                                          -------------        -----------------
               Total assets..........................       $  38,819                $  32,761  
                                                          -------------        -----------------
                                                          -------------        -----------------
                                                                                                
               LIABILITIES                                                                      
Current liabilities:                                                                            
     Note payable to bank............................       $       -                $      29  
     Long-term debt, current portion.................             151                      105  
     Capital lease obligation, current portion.......              42                       49  
     Accounts payable................................           1,307                    1,210  
     Accrued liabilities.............................           6,394                    5,190  
     Deferred revenue................................           5,462                    5,674  
                                                          -------------        -----------------
          Total current liabilities..................          13,356                   12,257  
                                                                                                
Long-term debt, less current portion.................             156                      194  
Capital lease obligations, less current portion......              24                       43  
Deferred revenue, less current portion...............             175                        -  
                                                          -------------        -----------------
          Total liabilities..........................          13,711                   12,494  
                                                          -------------        -----------------
                                                                                                
Commitments and contingencies                                                                   
                                                                                                
           STOCKHOLDERS' EQUITY                                                                 
Common stock, $.01 par value.                                                                   
Authorized 30,000 shares; issued and                                                           
outstanding 15,213 shares at June 30, 1998 and 
15,841 shares at December 31, 1997..................              152                      159  
Additional paid-in capital..........................           39,791                   51,797  
Treasury stock, at cost, 939 shares at                                                         
December 31, 1997...................................                -                  (11,555)  
Accumulated deficit.................................          (14,835)                 (20,134)    
                                                          -------------        -----------------
          Total stockholders' equity................           25,108                   20,267  
                                                          -------------        -----------------
               Total liabilities and stockholders' 
                 equity............................         $  38,819                $  32,761  
                                                          -------------        -----------------
                                                          -------------        -----------------
</TABLE>

           The accompanying notes are an integral part of the consolidated
                                financial statements

                                       -3-
<PAGE>

                                SUMMIT DESIGN, INC.
                       CONSOLIDATED STATEMENTS OF OPERATIONS
                       (In thousands, except per share data)
                                    (Unaudited)
<TABLE>
<CAPTION>
                                                    Three Months Ended               Six Months Ended
                                                         June 30                         June 30,
                                                 ------------------------         -------------------------
                                                   1998           1997               1998            1997
                                                 --------        --------         ---------       ---------
<S>                                              <C>             <C>              <C>             <C>
Revenue:
     Product licenses...................         $  8,574        $  5,611         $  16,775       $  10,507
     Maintenance and services...........            2,347           1,444             4,411           2,926
     Other..............................               91             125               183             267
                                                 --------        --------         ---------       ---------
          Total revenue.................           11,012           7,180            21,369          13,700

Cost of revenue:
     Product licenses...................              205             164               484             349
     Maintenance and services...........              279             142               505             252
                                                 --------        --------         ---------       ---------
          Total cost of revenue.........              484             306               989             601
                                                 --------        --------         ---------       ---------
               Gross profit.............           10,528           6,874            20,380          13,099

Operating expenses:
     Research and development...........            2,428           1,675             4,807           3,127
     Sales and marketing................            3,257           2,584             6,305           5,115
     General and administrative.........            1,344             881             2,442           2,061
                                                 --------        --------         ---------       ---------
          Total operating expenses......            7,029           5,140            13,554          10,303
     

Income from operations..................            3,499           1,734             6,826           2,796

Other income (expense), net.............              204             229               492             440
                                                 --------        --------         ---------       ---------
Income before income taxes..............            3,703           1,963             7,318           3,236
Income tax provision....................            1,041             100             2,019             180
                                                 --------        --------         ---------       ---------
Net income..............................         $  2,662         $ 1,863         $   5,299       $   3,056
                                                 --------        --------         ---------       ---------
                                                 --------        --------         ---------       ---------

Earnings per share:
          Basic.........................        $    0.18        $   0.13        $     0.35       $    0.22
                                                 --------        --------         ---------       ---------
                                                 --------        --------         ---------       ---------
          Diluted.......................        $    0.16        $   0.12        $     0.33       $    0.20
                                                 --------        --------         ---------       ---------
                                                 --------        --------         ---------       ---------
Number of shares used in computing
earnings per share:
          Basic.........................           15,058          14,167            14,984          14,137
          Diluted.......................           16,285          14,965            16,240          15,000
</TABLE>

           The accompanying notes are an integral part of the consolidated
                                financial statements

                                       -4-
<PAGE>

                                SUMMIT DESIGN, INC.
                       CONSOLIDATED STATEMENTS OF CASH FLOWS
                                   (In thousands)
                                    (Unaudited)
<TABLE>
<CAPTION>
                                                                              Six Months Ended
                                                                                  June 30,
                                                                       ------------------------------
                                                                          1998                 1997
                                                                       --------             ---------
<S>                                                                    <C>                  <C>
Cash flows from operating activities:
     Net income.................................................       $  5,299             $ 3,056
     Adjustments to reconcile net income to net cash 
      provided by operating activities:
        Depreciation and amortization...........................            781                 403
        Loss on asset disposition...............................              -                   1
        Deferred taxes..........................................            (81)                  -
        Equity in losses and elimination of intercompany
           profits of unconsolidated joint venture..............            350                   -
        Changes in assets and liabilities:
        Accounts receivable.....................................           (541)               (354)
        Prepaid expenses and other..............................            217                  47
        Accounts payable........................................             97                 155
        Accrued liabilities.....................................          1,204                 165
        Deferred revenue........................................            (37)               (370)
        Other, net..............................................            131                 104
                                                                       --------             -------
     Net cash provided by operating activities..................          7,420               3,207
                                                                       --------             -------
Cash flows from investing activities:
     Additions to furniture and equipment.......................         (1,045)               (751)
     Loan to joint venture......................................           (750)                  - 
     Notes receivable, net......................................           (325)               (425)
                                                                       --------             -------
        Net cash used in investing activities...................         (2,120)             (1,176)
                                                                       --------             -------
Cash flows from financing activities:
     Issuance of common stock, net of issuance costs............          1,046                 303
     Tax benefit of option exercises............................            825                   - 
     Payments to acquire treasury stock.........................         (2,329)                  - 
     Principal payments of debt obligations.....................            (21)                (81)
     Principal payments of capital lease obligations............            (26)                (49)
                                                                       --------             -------
        Net cash (used in) provided by financing activities.....           (505)                173
                                                                       --------             -------
        Increase in cash and cash equivalents...................          4,795               2,204
Cash and cash equivalents, beginning of period..................         19,973              19,801
                                                                       --------             -------
Cash and cash equivalents, end of period........................       $ 24,768             $22,005
                                                                       --------             -------
                                                                       --------             -------

Supplemental disclosure of cash flow information:
     Cash paid during the period for:
        Interest                                                       $      3             $     9
        Income taxes                                                        667                  38
Supplemental disclosure of non-cash financing activities:
      Retirement of treasury stock                                     $ 11,555             $     -
</TABLE>

          The accompanying notes are an integral part of the consolidated
                                financial statements

                                       -5-
<PAGE>

                                SUMMIT DESIGN, INC.
                     Notes to Consolidated Financial Statements
                                    (Unaudited)


1.   BASIS OF PRESENTATION

The accompanying unaudited financial statements have been prepared by Summit 
Design, Inc. ("Summit" or "the Company") in accordance with 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 in accordance with such rules and regulations. In the 
opinion of management, the accompanying unaudited financial statements 
reflect all adjustments, consisting only of normal recurring adjustments, 
necessary to present fairly the financial position of the Company, and its 
results of operations and cash flows. These financial statements should be 
read in conjunction with the audited financial statements and notes thereto 
for the years ended December 31, 1997, 1996 and 1995 included in the 
Company's Form 10-K filed for December 31, 1997.

The results of operations for the six months ended June 30, 1998 are not 
necessarily indicative of the results that may be expected for the year ended 
December 31, 1998 or any other future interim period, and the Company makes 
no representations related thereto.

2.   ACQUISITION OF PROSOFT OY

On June 30, 1998, the Company acquired ProSoft Oy ("ProSoft"), a Company 
located in Finland. ProSoft develops software tools used to verify embedded 
systems software prior to the availability of a hardware prototype. The 
aggregate consideration for the acquisition (including shares of common stock 
reserved for issuance upon exercise of ProSoft options which were exchanged 
for options of the Company) was 248,334 shares of common stock. The 
transaction was accounted for as a "pooling of interests" in accordance with 
generally accepted accounting principles. In compliance with such principles, 
the Company's financial statements have been restated to include the accounts of
ProSoft as if the acquisition had occurred at the beginning of the first 
period presented. The effect of the combination did not have a material 
impact on the net sales and net income of the combined entity.
 
3.   SOFTWARE REVENUE RECOGNITION

During the first quarter of 1998, the Company adopted Statements of Position 
(SOP) 97-2, "Software Revenue Recognition" and 98-4, "Deferral of the 
Effective Date of a Provision of SOP 97-2, "Software Revenue Recognition."" 
The provisions of SOP's 97-2 and 98-4 have been applied to transactions 
entered into beginning January 1, 1998. SOP 97-2 generally requires revenue 
earned on software arrangements involving multiple elements to be allocated 
to each element based on vendor-specific objective evidence (VSOE) of the 
fair value of the various elements in a multiple element arrangement Revenue 
from the sale of software licenses is recognized at the later of the time of 
shipment or satisfaction of all acceptance terms. The revenue allocated to 
maintenance is recognized ratably over the term of the maintenance agreement 
and revenue allocated to services is recognized as the services are 
performed. 

SOP 98-4 defers for one year, the application of several paragraphs and 
examples in SOP 97-2 that limit the definition of vendor specific objective 
evidence (VSOE) of the fair value of various elements in a multiple element 
arrangement. 

The Company analyzed the elements included in its multiple element 
arrangements and determined that the Company has sufficient evidence to 
allocate revenue to the license and maintenance components of its product 
licenses. The adoption of SOP's 97-2 and 98-4 did not have a significant 
effect on revenue recognized for the three and six month periods ending June 
30, 1998.

                                       -6-
<PAGE>

                                SUMMIT DESIGN, INC.
                     Notes To Consolidated Financial Statements
                                    (Unaudited)

4.   BALANCE SHEET COMPONENTS (IN THOUSANDS)


<TABLE>
<CAPTION>
                                                       June 30, 1998         December 31, 1997
                                                       -------------         -----------------
                                                        (Unaudited)
<S>                                                    <C>                   <C>

Accounts receivable:
     Trade receivables...........................          $   6,061             $    5,723 
     Less allowance for doubtful accounts........               (389)                  (592)
                                                           ---------             ----------
                                                           $   5,672             $    5,131
                                                           ---------             ----------
                                                           ---------             ----------
Furniture and equipment:
     Office furniture equipment..................          $     930             $      596
     Computer equipment..........................              4,377                  3,679
     Leasehold improvements......................                 79                     66
                                                           ---------             ----------
                                                               5,386                  4,341
Less: accumulated depreciation and amortization..             (2,178)                (1,643)
                                                           ---------             ----------
                                                           $   3,208              $   2,698
                                                           ---------             ----------
                                                           ---------             ----------
Accrued expenses:
     Payroll and related benefits................         $    3,369              $   2,888
     Sales and marketing.........................                985                    435
     Accounting and legal........................                226                    260
     Federal and state income taxes payable......              1,343                    819
     Sales taxes payable.........................                 76                    114
     Other.......................................                395                    674
                                                           ---------             ----------
          Total accrued expenses.................         $    6,394              $   5,190
                                                           ---------             ----------
                                                           ---------             ----------
</TABLE>

                                       -7-
<PAGE>

                                SUMMIT DESIGN, INC.
                     Notes To Consolidated Financial Statements
                                    (Unaudited)

5.   RECONCILIATION OF EARNINGS PER SHARE

Basic earnings per share is computed using the weighted average number of 
common shares outstanding during the period. Diluted earnings per share is 
computed using the weighted average number of common and dilutive common 
equivalent shares outstanding during the period. Dilutive common equivalent 
shares consist of common stock issuable upon exercise of stock options using 
the treasury stock method. The following provides a reconciliation of the 
numerators and denominators of the basic and diluted per share computations:

<TABLE>
<CAPTION>
                                                   Three months ended        Six months ended
                                                         June 30,                June 30,
                                                   ------------------     --------------------
                                                      1998      1997         1998      1997   
                                                   ---------  ---------   ---------   --------
<S>                                                <C>        <C>         <C>         <C>
 Numerator:
    Net income                                     $   2,662  $   1,863   $   5,299   $  3,056
                                                   ---------  ---------   ---------   --------
                                                   ---------  ---------   ---------   --------
 Denominator:
    Denominator for basic earnings per share 
      weighted average shares                         15,058     14,167      14,984     14,137

    Effect of dilutive securities:
      Employee stock options                           1,227        798       1,256        863
                                                   ---------  ---------   ---------   --------

    Denominator for diluted earnings per share        16,285     14,965      16,240     15,000 
                                                   ---------  ---------   ---------   --------
                                                   ---------  ---------   ---------   --------
   

 Net income per share - basic                      $    0.18  $    0.13   $    0.35  $    0.22 
                                                   ---------  ---------   ---------   --------
                                                   ---------  ---------   ---------   --------

 Net income per share - diluted                    $    0.16  $    0.12   $    0.33  $    0.20 
                                                   ---------  ---------   ---------   --------
                                                   ---------  ---------   ---------   --------
</TABLE>

6.   IMPACT OF RECENTLY ISSUED ACCOUNTING STANDARDS

In June 1998, the FASB issued SFAS No. 133, "Accounting for Derivitive 
Instruments and Hedging Activities." SFAS No. 133 establishes accounting and 
reporting standards requiring that every derivitive instrument be recorded in 
the balance sheet as either an asset or liability measured at its fair value. 
SFAS No. 133 also requires that changes in the derivitive instrument's fair 
value be recognized currently in results of operations unless specific hedge 
accounting criteria are met. SFAS No. 133 is effective for fiscal years 
beginning after June 15, 1999. The Company does not expect SFAS No. 133 to 
have a material impact on its consolidated financial statements.

                                       -8-
<PAGE>

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.

                              SUMMIT DESIGN, INC.


                              By:  /s/ C. Albert Koob
                                   ------------------------------------
                                   C. Albert Koob
                                   Vice President - Finance, 
                                   Chief Financial Officer and Secretary
                                   (Principal Financial and Accounting
                                   Officer and Duly Authorized Officer)


Date:     August 31, 1998


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