EXHIBIT 5
                                                                      ---------
 
                        QUICKTURN DESIGN SYSTEMS, INC.
                              55 W. TRIMBLE ROAD
                          SAN JOSE, CALIFORNIA 95131
 
                                August 24, 1998
 
TO THE STOCKHOLDERS OF
QUICKTURN DESIGN SYSTEMS, INC.
 
Dear Stockholder:
 
  As you may be aware, Mentor Graphics Corporation ("Mentor") and its wholly-
owned subsidiary, MGZ Corp, commenced on August 12, 1998 an unsolicited tender
offer (the "Mentor Offer") for all of the Common Stock of Quickturn Design
Systems, Inc. ("Quickturn" or the "Company") at $12.125 per share. After
careful consideration, Quickturn's Board of Directors has voted to recommend
that stockholders reject as inadequate the unsolicited $12.125 per share
tender offer by Mentor.
 
                  YOUR BOARD RECOMMENDS THAT THE STOCKHOLDERS
                           REJECT THE MENTOR OFFER.
 
  After careful consideration of the Mentor Offer, your Board determined that
the Mentor Offer was inadequate and not in the best interests of the Company's
stockholders, that the Mentor Offer did not fully reflect the long-term value
of the Company, and that stockholder interests would be better served by the
Company continuing to pursue its business plan. In particular, the Board
determined that the Company's business plan offered the potential for
obtaining higher long-term benefits for the Company's stockholders than the
Mentor Offer. This determination was based on, among other things, the opinion
of the Company's financial advisers, Hambrecht & Quist LLC, that the Mentor
Offer is inadequate and the opportunities for business expansion and revenue
and earnings growth resulting from recently introduced products and from
products under development for use in the electronic design automation market
and in other related parts of the market.
 
  The enclosed Schedule 14D-9 describes your Board's decision to reject the
Mentor Offer and contains other important information relating to its
decision. We urge you to read it carefully.
 
  In rejecting the Mentor Offer, we have reaffirmed our continued confidence
in the Company's future and our determination that you, our stockholders, be
given every opportunity to participate fully in that future. Your Board of
Directors and I greatly appreciate your continued support and encouragement.
 
                                          Very truly yours,
 
                                          LOGO
                                          /s/ KEITH R. LOBO
 
                                          Keith R. Lobo
                                          PRESIDENT AND CHIEF EXECUTIVE
                                           OFFICER