Exhibit 99.1

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT TO SECTION
906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of Thoratec Corporation (the "Company")
on Form 10-Q for the period ending March 29, 2003 as filed with the Securities
and Exchange Commission on the date hereof (the "Report"), I, D. Keith Grossman,
Chief Executive Officer of the Company, certify, pursuant to 18 U.S.C. Section
1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002,
that:

      (1)   The Report fully complies with the requirements of section 13(a) or
            15(d) of the Securities Exchange Act of 1934; and

      (2)   The information contained in the Report fairly presents, in all
            material respects, the financial condition and result of operations
            of the Company.


  By:        /s/   D. Keith Grossman          Date: May 13, 2003
             ----------------------------
  Name:      D. Keith Grossman
  Title:     Chief Executive Officer

In connection with the Quarterly Report of Thoratec Corporation (the "Company")
on Form 10-Q for the period ending March 29, 2003 as filed with the Securities
and Exchange Commission on the date hereof (the "Report"), I, M. Wayne Boylston,
Chief Financial Officer of the Company, certify, pursuant to 18 U.S.C. Section
1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002,
that:

      (1)   The Report fully complies with the requirements of section 13(a) or
            15(d) of the Securities Exchange Act of 1934; and

      (2)   The information contained in the Report fairly presents, in all
            material respects, the financial condition and result of operations
            of the Company.


  By:        /s/   M. Wayne Boylston           Date: May 13, 2003
             ----------------------------
  Name:      M. Wayne Boylston
  Title:     Chief Financial Officer