Exhibit 99.1


                            EXPLANATION OF RESPONSES

(1)  Pursuant to a Note Purchase Agreement, dated as of December 4, 2003, by and
     among Micro Therapeutics, Inc. (the "Company"), Warburg, Pincus Equity
     Partners, L.P., a Delaware limited partnership (together with three
     affiliated entities, "WPEP"), and the other investors named therein (the
     "Note Purchase Agreement"), WPEP agreed to purchase an aggregate principal
     amount of $11,700,000 of exchangeable promissory notes from the Company
     (the "Exchangeable Notes"). The Exchangeable Notes accrued interest at 7%
     per annum, which interest compounded quarterly. On January 30, 2004, WPEP
     exchanged all of the Exchangeable Notes held by WPEP for 4,333,518 shares
     of common stock, of which Warburg, Pincus Equity Partners, L.P. received
     4,095,174 shares and three entities affiliates with Warburg, Pincus Equity
     Partners, L.P. received an aggregate of 238,344 shares. The number of
     shares of common stock received by WPEP was determined by dividing (i) the
     aggregate principal balance on such Exchangeable Notes plus accrued and
     unpaid interest thereon at January 30, 2004 by (ii) $2.73.

(2)  The sole general partner of WPEP is Warburg Pincus & Co., a New York
     general partnership ("WP"). Warburg Pincus LLC, a New York limited
     liability company ("WP LLC"), manages WPEP. By reason of the provisions of
     Rule 16a-1 of the Securities Exchange Act of 1934, as amended, WPEP, WP and
     WP LLC may be deemed to be the beneficial owner of an indeterminate portion
     of the 24,336,759 shares of the Company's common stock owned by Micro
     Investment, LLC, a Delaware limited liability company, which is indirectly
     controlled by WPEP.