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                                                                     EXHIBIT 5.1
       
                             FOLEY, HOAG & ELIOT LLP
                             One Post Office Square
                           Boston, Massachusetts 02109


                                        May 21, 1997

Cayenne Software, Inc.
8 New England Executive Park
Burlington, MA  01803

Ladies and Gentlemen:

       We are familiar with the Registration Statement on Form S-8 transmitted
for filing by the Company today (the "S-8 Registration Statement") relating to
2,000,000 shares (the "Shares") of the Company's Common Stock to be offered
puruant to the Company's Amended 1996 Incentive and Nonqualified Stock Option
Plan (the "Plan").

       We are familiar with the Company's Articles of Organization and all
amendments thereto, its By-Laws and all amendments thereto, the records of all
meetings and consents of its Board of Directors and of its stockholders, and its
stock records. We have examined such other records and documents as we deemed
necessary or appropriate for purposes of rendering this opinion.

       Based upon the foregoing, we are of the opinion that (a) the Company has
corporate power adequate for the issuance of the Shares in the manner set forth
in the S-8 Registration Statement; and (b) upon issuance upon exercise of
options granted under the Plan in against payment of the specified exercise
prices therefor, the Shares will be legally issued, fully paid and
non-assessable.

       We consent to the filing of this opinion as an exhibit to the S-8
Registration Statement.

                                              Very truly yours,



                                              FOLEY, HOAG & ELIOT LLP
                                       
                                              By: /s/ David W. Walker
                                                  ----------------------------
                                                  a Partner








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