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                                                                     EXHIBIT 5.1

                   OPINION OF BROBECK, PHLEGER & HARRISON LLP

                                September 6, 2001

Illumina, Inc.
9390 Towne Centre Drive, Suite 200
San Diego, California  92121

        Re:     Registration Statement on Form S-8 for 2,458,946 Shares of
                Common Stock

Ladies and Gentlemen:

                We have acted as counsel to Illumina, Inc., a Delaware
corporation (the "Company"), in connection with the registration on Form S-8
(the "Registration Statement") under the Securities Act of 1933, as amended, of
(i) an additional reserve of 1,500,000 shares of common stock (the "Shares") for
issuance under the Company's 2000 Stock Plan, as amended to date (the "Option
Plan"), and (ii) an additional reserve of 958,946 Shares authorized for issuance
under the Company's 2000 Employee Stock Purchase Plan, as amended to date (the
"Purchase Plan" and together with the Option Plan, the "Plans").

                This opinion is being furnished in accordance with the
requirements of Item 8 of Form S-8 and Item 601(b)(5)(i) of Regulation S-K.

                We have reviewed the Company's charter documents and the
corporate proceedings taken by the Company in connection with the establishment
of the Plans. Based on such review, we are of the opinion that, if, as and when
the Shares have been issued and sold (and the consideration therefor received)
in accordance with the Registration Statement and pursuant to either (i) the
provisions of option agreements duly authorized under the Option Plan or (ii)
duly authorized stock purchase rights granted and exercised under the Purchase
Plan and in accordance with the Registration Statement, such Shares will be duly
authorized, legally issued, fully paid and nonassessable.

                We consent to the filing of this opinion letter as Exhibit 5.1
to the Registration Statement.

                This opinion letter is rendered as of the date first written
above and we disclaim any obligation to advise you of facts, circumstances,
events or developments which hereafter may be brought to our attention and which
may alter, affect or modify the opinion expressed herein. Our opinion is
expressly limited to the matters set forth above and we render no opinion,
whether by implication or otherwise, as to any other matters relating to the
Company, the Plans or the Shares.

                                            Very truly yours,
                                            BROBECK, PHLEGER & HARRISON LLP