Exhibit 5.1

OPINION OF WILSON SONSINI GOODRICH & ROSATI, PROFESSIONAL CORPORATION

July 27, 2005

Ventana Medical Systems, Inc.

1910 Innovation Drive

Tucson, AZ 85755

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 around July 27, 2005 (the “Registration Statement”), in connection with the registration under the Securities Act of 1933, as amended, of an aggregate of 2,500,000 additional shares of your common stock, par value $0.001 per share (the “Shares”), (i) 2,000,000 shares of which are to be issued pursuant to options under the 2005 Equity Incentive Plan and (ii) 500,000 shares of which are to be issued pursuant to the 2005 Employee Stock Purchase Plan (collectively, the “Plans”). As your counsel in connection with this transaction, we have examined the proceedings taken and are familiar with the proceedings proposed to be taken by you in connection with the issuance and sale of the Shares pursuant to the Plans.

It is our opinion that, when issued and sold in the manner described in the Plans and pursuant to the agreements which accompany each grant under the Plans, the Shares 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 and any amendments thereto.

Very truly yours,

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation

/s/ WILSON SONSINI GOODRICH & ROSATI, P.C.