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                                                                     EXHIBIT 5.1


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



                                        April 4, 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-3 transmitted
for filing by the Company today (the "S-8 Registration Statement") relating to
1,889,195 shares (the "Shares") of the Company's Common Stock to be offered and
sold by certain holders of Series B Convertible Preferred Stock (the "Preferred
Stock") and Warrants of the Company (the "Selling Stockholders").

         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-3 Registration Statement; (b) the Company has taken all necessary
corporate action required to authorize the issuance and sale of the Shares to
the Selling Stockholders upon conversion of Preferred Stock or exercise of
Warrants; and (c) upon issuance in accordance with the aplicable provisions of
the Preferred Stock or the Warrants, 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-3
Registration Statement.
                                             Very truly yours,

                                             FOLEY, HOAG & ELIOT


                                             By: /s/ David W. Walker
                                                  a Partner