EXHIBIT 10.31

                             FIFTH ADDENDUM TO LEASE
                                 BY AND BETWEEN
                     THE JOSEPH AND EDA PELL REVOCABLE TRUST
                                ("THE LANDLORD")
                                       AND
                      FAIR, ISAAC AND COMPANY, INCORPORATED
                                 ("THE TENANT")


         This  Fifth   Addendum  to  Lease  dated   November  22,  1996  ("Fifth
Addendum"),  is hereby attached to and incorporated into and made a part of that
Lease dated October 11, 1993,  by and between The Joseph and Eda Pell  Revocable
Trust and Fair,  Isaac and Company,  Incorporated as amended by a First Addendum
to Lease; a Second Addendum to Lease dated January 31, 1994; a Third Addendum to
Lease dated December 15, 1994; and Fourth  Addendum to Lease dated April 3, 1995
("the Lease"). The parties agree to the following terms and conditions set forth
herein below:

                                    Recitals

         A.  Landlord  and Tenant are  parties  to the Lease  pursuant  to which
Landlord  leased  to  Tenant  and  Tenant  leased  from  Landlord  office  space
containing  approximately  11,875 rentable square feet and 10,642 useable square
feet  comprising  part of the of the First Floor of that certain office building
known as Regency  Center I ("Regency  I")  located at 100 Smith Ranch Road,  San
Rafael, California ("the Premises").

         B. Landlord is currently constructing a building adjacent to Regency I,
which new building will be known as Regency  Center II, and which will be leased
in its  entirety  to Tenant  pursuant to an Office  Building  Lease of even date
herewith.

         C. Landlord and Tenant now desire to amend the Lease as hereinafter set
forth.

         D. The capitalized terms herein, unless otherwise indicated, shall have
the meanings ascribed to them in the Lease.

         NOW, THEREFORE,  for good and valuable  consideration,  the receipt and
sufficiency of which are hereby acknowledged, the parties agree as follows:

         1. Paragraph 3 ("Right of First  Opportunity") of the First Addendum to
Lease is hereby deleted in its entirety.

         2. Subparagraph A and  sub-subparagraph  (i) of Paragraph 2 ("Option to
Extend")  of the First  Addendum to Lease are hereby  deleted and the  following
substituted therefor:

                  "A. Landlord grants to Tenant the option to extend the term of
                  this Lease for the Existing Premises for one (1) ten (10) year
                  period commencing upon expiration of the Term for the lease of
                  the  Existing  Premises,  upon  each and all of the  following
                  terms and conditions:"

                           "(i) Tenant gives to Landlord  and Landlord  receives
                           notice of the  exercise  of the option to extend this
                           Lease for said  additional  term no later than twelve
                           (12) months prior to the time that the option  period
                           would  commence  if the option were  exercised,  time
                           being of the  essence.  If said  notification  of the
                           exercise of said option is not so given and received,
                           this option shall automatically expire;"

         3. Except as set forth herein, the Lease shall remain unmodified and in
full force and effect.  Should  there by any  conflict  between the terms of the
Lease and the terms of this Fifth  Addendum,  the terms of this  Fifth  Addendum
shall control.

         IT WITNESS  WHEREOF,  the parties have executed this Fifth  Addendum to
Lease as of the date first written above.


                                         The Joseph and Eda Pell RevocableTrust



                                         By:  /s/ Joseph Pell
                                              --------------------------------
                                         Joseph Pell, Trustee



                                         By:  /s/ Eda Pell
                                              --------------------------------
                                         Eda Pell, Trustee



                                         Fair, Isaac and Company, Incorporated



                                         By:  /s/ Michael C. Gordon
                                              --------------------------------
                                         Its: Vice President