UNITED STATES
                SECURITIES AND EXCHANGE COMMISSION
                      Washington, D.C. 20549


                           SCHEDULE 13G

             Under the Securities Exchange Act of 1934

                        (Amendment No. 6)*

                        KILROY REALTY CORP

                          (Name of Issuer)

                           COMMON STOCK

                   (Title of Class of Securities)


                             49427F108

                          (CUSIP Number)

                         February 29, 2008

        (Date of Event which Requires Filing of Statement)

Check the appropriate box to designate the Rule pursuant to which
this Schedule is filed:

[x] Rule 13d - 1(b)

    Rule 13d - 1(c)

    Rule 13d - 1(d)

*The remainder of this cover page shall be filled out for a
reporting person's initial filing on this form with respect to the
subject class of securities, and for any subsequent amendment
containing information which would alter disclosures provided in a
prior cover page.

The information required on the remainder of this page shall not be
deemed to be "filed" for the purpose of Section 18 of the
Securities Exchange Act of 1934 ("Act") or otherwise subject to the
liabilities of that section of the Act but shall be subject to all
other provisions of the Act (however, see the Notes.)

                  (Continued on following page(s)
                        Page 1 of 6 Pages
CUSIP NO. 49427F108              13G           Page 2 of 6 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             750,740

Beneficially    6  Shared Voting Power
                **
Owned By Each      NONE

Reporting       7  Sole Dispositive Power
                **
Person             3,357,637

With            8  Shared Dispositive Power

                   NONE

9   Aggregate Amount Beneficially Owned by Each Reporting Person

    3,357,637

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

    10.0%

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 6

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:

           12200 W. Olympic Blvd., Ste. 200, Los Angeles, CA 90064


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: 49427F108


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
CUSIP 49427F108
PAGE 4 OF 6

Item 4     Ownership                           Deemed
                                           Outstanding
                                               And
                                           Beneficially
                              Units       Owned Directly
                              Deemed        Subject to
                           Beneficially     Warrants &
                              Owned         Conversion
                             Directly       Privileges    Total
                           ____________   ______________ _______

(1)  WITH RESPECT TO
     PRICE ASSOCIATES
     (includes shares
     reported in
     (2) below):

     (a)Amount
         Beneficially
         Owned . . . . . .   2,875,570     482,067       3,357,637
     (b)Percent of
         Class . . . . . . . . . . . . . . . . . . . . . .  10.0%
     (c)Number of
         units as
         to which
         such
         person has: . . .

        (i)  *sole power
             to vote or
             to direct
             the vote . . .    662,250      88,490         750,740

       (ii)  *shared power
             to vote or
             to direct
             the vote . . .       -0-        -0-          -0-

      (iii)  *sole power
             to dispose or
             to direct the
             disposition
             of . . . . .     2,875,570     482,067       3,357,637

       (iv)  *shared power
             to dispose or
             to direct the
             disposition
             of . . . . .         -0-        -0-          -0-
SCHEDULE 13G
PAGE 5 OF 6

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

  X     Not Applicable.

_____   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 6 OF 6


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 this
          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:  March 10, 2008

              T. ROWE PRICE ASSOCIATES, INC.



              By: /s/ Henry H. Hopkins
                  Henry H. Hopkins, 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.

02/29/2008