Exhibit 5.1
Wilson Sonsini Goodrich & Rosati, P.C.
San Francisco, California


                                October 11, 2001


TheraSense, Inc.
1360 South Loop Road
Alameda, CA 94502

     Re:    Registration Statement on Form S-8

Ladies and Gentlemen:

     We have examined the Registration Statement on Form S-8 to be filed by you
with the Securities and Exchange Commission on or about October 11 2001 (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, of 7,085,530 shares of your Common Stock
reserved for issuance under the 2001 Stock Plan (the "2001 Plan"). As your legal
counsel, we have examined the proceedings taken and are familiar with the
proceedings proposed to be taken by you in connection with the sale and issuance
of such Common Stock under the 2001 Plan.

     It is our opinion that, when issued and sold in the respective manners
referred to in the 2001 Plan and pursuant to the agreements which accompany the
2001 Plan, the Common Stock issued and sold thereby will be legally and validly
issued, fully paid and non-assessable.

     We consent to the use of this opinion as an exhibit to the Registration
Statement and further consent to the use of our name wherever appearing in the
Registration Statement, including any amendments thereto.

                                     Very truly yours,

                                     WILSON SONSINI GOODRICH & ROSATI
                                     Professional Corporation


                                     /s/ Wilson Sonsini Goodrich & Rosati, P.C.