EXHIBIT 5.1
                                                                     -----------







                                             September 30, 1998



Fair, Isaac and Company, Incorporated
120 North Redwood Drive
San Rafael, CA 94903


Re:  Registration Statement on Form S-8 for the Fair, Isaac and Company,
     Incorporated 1992 Long-Term Incentive Plan


Ladies and Gentlemen:

     With reference to the Registration Statement on Form S-8 to be filed by
Fair, Isaac and Company, Incorporated, a Delaware corporation (the "Company"),
with the Securities and Exchange Commission under the Securities Act of 1933
relating to 538,490 additional shares of the Company's Common Stock issuable
pursuant to the 1992 Long-Term Incentive Plan (the "Plan"), it is our opinion
that such shares of the Common Stock of the Company, when issued and sold in
accordance with the Plan, will be legally issued, fully paid and nonassessable.

     We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as Exhibit 5.1 and Exhibit 23.1 to the Registration
Statement.

                                    Very truly yours,

                                    /s/ PILLSBURY MADISON & SUTRO LLP

E-9372