Exhibit 10.20(a)

                                   EXHIBIT B-1

                      AMENDED AND RESTATED PROMISSORY NOTE

$17,500                                                            March 1, 2002


This AMENDED AND RESTATED PROMISSORY NOTE amends and restates in its entirety
the PROMISSORY NOTE dated July 30, 1998 originally delivered by the undersigned
to TheraSense, Inc., a Delaware corporation (the "Company"). Any capitalized
terms not defined herein shall have the meaning given to them in the Repurchase
Agreement between the Company and the undersigned dated July 30, 1998.

For value received, the undersigned promises to pay to the Company, or order, at
its principal office the principal sum of $17,500 without interest. Said
principal shall be due on the earlier of April 6, 2003, or (ii) the
undersigned's termination of employment with or services to the Company.

Should suit be commenced to collect this Note or any portion thereof, such sum
as the Court may deem reasonable shall be added hereto as attorneys' fees. The
maker waives presentment for payment, protest, notice of protest, and notice of
non-payment of this Note.

This Note is secured by a pledge of 62,500 (post-split) shares of Common Stock
of the Company, pursuant to the provisions of the Repurchase Agreement between
the Company and the undersigned executed on July 30, 1998.

The holder of this Note shall have full recourse against the maker, and shall
not be required to proceed against the Shares or other collateral securing this
Note in the event of default.

                                      /s/  CHARLES T. LIAMOS
                                      -----------------------------------------
                                      Charles Liamos