ILMN Illumina Inc

CUSIP NO. 452327109 13G Page 2 of 5 Pages

1 Name of Reporting Person
 S.S. or I.R.S. Identification No. of Above Person

 T. ROWE PRICE ASSOCIATES, INC.
 52-0556948

2 Check the Appropriate Box if a Member of a Group*

 (a) ____
 NOT APPLICABLE (b) ____

3 SEC Use Only

 ______________________________

4 Citizenship or Place of Organization

 MARYLAND

Number of 5 Sole Voting Power
 **
Shares 1,238,480

Beneficially 6 Shared Voting Power
 **
Owned By Each -0-

Reporting 7 Sole Dispositive Power
 **
Person 6,044,680

With 8 Shared Dispositive Power

 -0-

9 Aggregate Amount Beneficially Owned by Each Reporting Person

 6,044,680

10 Check Box if the Aggregate Amount in Row (9) Excludes Certain
 Shares*

 NOT APPLICABLE

11 Percent of Class Represented by Amount in Row 9

 4.9%

12 Type of Reporting Person*

 IA
 *SEE INSTRUCTION BEFORE FILLING OUT!
 **Any shares reported in Items 5 and 6 are also
 reported in Item 7.
SCHEDULE 13G
PAGE 3 OF 5

Item 1(a) Name of Issuer:

 Reference is made to page 1 of this Schedule 13G

Item 1(b) Address of Issuer's Principal Executive Offices:

 9885 TOWNE CENTRE DR, SAN DIEGO, CA 92121

Item 2(a) Name of Person(s) Filing:

 (1) T. Rowe Price Associates, Inc. ("Price
 Associates")

 (2) _______________________________________

_____ Attached as Exhibit A is a copy of an agreement between
 the Persons Filing (as specified hereinabove) that this
 Schedule 13G is being filed on behalf of each of them.

Item 2(b) Address of Principal Business Office:

 100 E. Pratt Street, Baltimore, Maryland 21202

Item 2(c) Citizenship or Place of Organization:

 (1) Maryland

 (2) ________

Item 2(d) Title of Class of Securities:

 Reference is made to page 1 of this Schedule 13G

Item 2(e) CUSIP Number: 452327109

Item 3 The person filing this Schedule 13G is an:

 X Investment Adviser registered under Section 203 of the
 Investment Advisers Act of 1940

_____ Investment Company registered under Section 8 of the
 Investment Company Act of 1940


Item 4 Reference is made to Items 5-11 on page 2 of this
 Schedule 13G.
SCHEDULE 13G
PAGE 4 OF 5

Item 5 Ownership of Five Percent or Less of a Class.

 Not Applicable.

 X This statement is being filed to report the fact that, as
 of the date of this report, the reporting person(s) has
 (have) ceased to be the beneficial owner of more than five
 percent of the class of securities.

Item 6 Ownership of More than Five Percent on Behalf of Another
 Person

 (1) Price Associates does not serve as custodian of the
 assets of any of its clients; accordingly, in each
 instance only the client or the client's custodian or
 trustee bank has the right to receive dividends paid
 with respect to, and proceeds from the sale of, such
 securities.

 The ultimate power to direct the receipt of dividends
 paid with respect to, and the proceeds from the sale
 of, such securities, is vested in the individual and
 institutional clients which Price Associates serves as
 investment adviser. Any and all discretionary
 authority which has been delegated to Price Associates
 may be revoked in whole or in part at any time.

 Except as may be indicated if this is a joint filing
 with one of the registered investment companies
 sponsored by Price Associates which it also serves as
 investment adviser ("T. Rowe Price Funds"), not more
 than 5% of the class of such securities is owned by
 any one client subject to the investment advice of
 Price Associates.

 (2) With respect to securities owned by any one of the T.
 Rowe Price Funds, only State Street Bank and Trust
 Company, as custodian for each of such Funds, has the
 right to receive dividends paid with respect to, and
 proceeds from the sale of, such securities. No other
 person is known to have such right, except that the
 shareholders of each such Fund participate
 proportionately in any dividends and distributions so
 paid.

Item 7 Identification and Classification of the Subsidiary Which
 Acquired the Security Being Reported on By the Parent
 Holding Company.

 Not Applicable.

Item 8 Identification and Classification of Members of the Group.

 Not Applicable.
SCHEDULE 13G
PAGE 5 OF 5

Item 9 Notice of Dissolution of Group.

 Not Applicable.

Item 10 Certification.

 By signing below I (we) certify that, to the best of my
 (our) knowledge and belief, the securities referred to
 above were acquired in the ordinary course of business
 and were not acquired for the purpose of and do not have
 the effect of changing or influencing the control of the
 issuer of such securities and were not acquired in
 connection with or as a participant in any transaction
 having such purpose or effect. T. Rowe Price
 Associates, Inc. hereby declares and affirms that the
 filing of Schedule 13G shall not be construed as an
 admission that Price Associates is the beneficial owner
 of the securities referred to, which beneficial
 ownership is expressly denied.

 Signature.

 After reasonable inquiry and to the best of my (our)
 knowledge and belief, I (we) certify that the
 information set forth in this statement is true,
 complete and correct.


 Dated: May 10, 2011


 T. ROWE PRICE ASSOCIATES, INC.



 By: /s/ David Oestreicher
 David Oestreicher, Vice President



Note: This Schedule 13G, including all exhibits, must be filed
 with the Securities and Exchange Commission, and a copy
 hereof must be sent to the issuer by registered or
 certified mail not later than February 14th following
 the calendar year covered by the statement or within the
 time specified in Rule 13d-1(b)(2), if applicable.

04/30/2011