EX-99.CERT
                               MFS SERIES TRUST XI

         CERTIFICATION PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT


    I, Tracy Atkinson, certify that:

    1. I have reviewed this report on Form N-CSR of MFS Series Trust XI;

    2. Based on my knowledge, this report does not contain any untrue statement
      of a material fact or omit to state a material fact necessary to make the
      statements made, in light of the circumstances under which such statements
      were made, not misleading with respect to the period covered by this
      report;

    3. Based on my knowledge, the financial statements, and other financial
      information included in this report, fairly present in all material
      respects the financial condition, results of operations, changes in net
      assets, and cash flows (if the financial statements are required to
      include a statement of cash flows) of the registrant as of, and for, the
      periods presented in this report;

    4. The registrant's other certifying officer and I are responsible for
      establishing and maintaining disclosure controls and procedures (as
      defined in Rule 30a-3(c) under the Investment Company Act of 1940) and
      internal control over financial reporting (as defined in Rule 30a-3(d)
      under the Investment Company Act of 1940) for the registrant and have:

       a. designed such disclosure controls and procedures, or caused such
          disclosure controls and procedures to be designed under our
          supervision, to ensure that material information relating to the
          registrant, including its consolidated subsidiaries, is made known to
          us by others within those entities, particularly during the period in
          which this report is being prepared;

       b. designed such internal control over financial reporting, or caused
          such internal control over financial reporting to be designed under
          our supervision, to provide reasonable assurance regarding the
          reliability of financial reporting and the preparation of financial
          statements for external purposes in accordance with generally accepted
          accounting principles;

       c. evaluated the effectiveness of the registrant's disclosure controls
          and procedures and presented in this report our conclusions about the
          effectiveness of the disclosure controls and procedures as of a date
          within 90 days prior to the filing date of this report based on such
          evaluation; and

       d. disclosed in this report any change in the registrant's internal
          control over financial reporting that occurred during the second
          fiscal quarter of the period covered by the report that has materially
          affected, or is reasonably likely to materially affect, the
          registrant's internal control over financial reporting; and

    5. The registrant's other certifying officer and I have disclosed to the
      registrant's auditors and the audit committee of the registrant's board of
      directors (or persons performing the equivalent functions):

       a. all significant deficiencies and material weaknesses in the design or
          operation of internal control over financial reporting which are
          reasonably likely to adversely affect the registrant's ability to
          record, process, summarize, and report financial information; and

       b. any fraud, whether or not material, that involves management or other
          employees who have a significant role in the registrant's internal
          control over financial reporting.



Date:  November 23, 2005              TRACY ATKINSON
       -----------------              -----------------------------------
                                      Tracy Atkinson
                                      Treasurer (Principal Financial Officer
                                      and Accounting Officer)

                                                                      EX-99.CERT
                               MFS SERIES TRUST XI

         CERTIFICATION PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT


    I, Maria F. Dwyer, certify that:

    1. I have reviewed this report on Form N-CSR of MFS Series Trust XI;

    2. Based on my knowledge, this report does not contain any untrue statement
       of a material fact or omit to state a material fact necessary to make the
       statements made, in light of the circumstances under which such
       statements were made, not misleading with respect to the period covered
       by this report;

    3. Based on my knowledge, the financial statements, and other financial
       information included in this report, fairly present in all material
       respects the financial condition, results of operations, changes in net
       assets, and cash flows (if the financial statements are required to
       include a statement of cash flows) of the registrant as of, and for, the
       periods presented in this report;

    4. The registrant's other certifying officer and I are responsible for
       establishing and maintaining disclosure controls and procedures (as
       defined in Rule 30a-3(c) under the Investment Company Act of 1940) and
       internal control over financial reporting (as defined in Rule 30a-3(d)
       under the Investment Company Act of 1940) for the registrant and have:

       a. designed such disclosure controls and procedures, or caused such
          disclosure controls and procedures to be designed under our
          supervision, to ensure that material information relating to the
          registrant, including its consolidated subsidiaries, is made known to
          us by others within those entities, particularly during the period in
          which this report is being prepared;

       b. designed such internal control over financial reporting, or caused
          such internal control over financial reporting to be designed under
          our supervision, to provide reasonable assurance regarding the
          reliability of financial reporting and the preparation of financial
          statements for external purposes in accordance with generally accepted
          accounting principles;

       c. evaluated the effectiveness of the registrant's disclosure controls
          and procedures and presented in this report our conclusions about the
          effectiveness of the disclosure controls and procedures as of a date
          within 90 days prior to the filing date of this report based on such
          evaluation; and

       d. disclosed in this report any change in the registrant's internal
          control over financial reporting that occurred during the second
          fiscal quarter of the period covered by the report that has materially
          affected, or is reasonably likely to materially affect, the
          registrant's internal control over financial reporting; and

    5. The registrant's other certifying officer and I have disclosed to the
       registrant's auditors and the audit committee of the registrant's board
       of directors (or persons performing the equivalent functions):

       a. all significant deficiencies and material weaknesses in the design or
          operation of internal control over financial reporting which are
          reasonably likely to adversely affect the registrant's ability to
          record, process, summarize, and report financial information; and

       b. any fraud, whether or not material, that involves management or other
          employees who have a significant role in the registrant's internal
          control over financial reporting.



Date: November 23, 2005               MARIA F. DWYER
      -----------------               -----------------------------------
                                      Maria F. Dwyer
                                      President (Principal Executive Officer)

                                                                   EX-99.906CERT
                               MFS SERIES TRUST XI

         CERTIFICATION PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT

    I, Tracy Atkinson, certify that, to my knowledge:

    1. The Form N-CSR (the "Report") of MFS Series Trust XI (the "Registrant")
       fully complies for the period covered by the Report with the requirements
       of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

    2. The information contained in the Report fairly presents, in all material
       respects, the financial condition and results of operations of the
       Registrant.



Date: November 23, 2005               TRACY ATKINSON
      -----------------               -----------------------------------
                                      Tracy Atkinson
                                      Treasurer (Principal Financial Officer
                                      and Accounting Officer)


A SIGNED ORIGINAL OF THIS WRITTEN STATEMENT REQUIRED BY SECTION 906 HAS BEEN
PROVIDED TO THE REGISTRANT AND WILL BE RETAINED BY THE REGISTRANT AND FURNISHED
TO THE SECURITIES AND EXCHANGE COMMISSION OR ITS STAFF UPON REQUEST.


                                                                   EX-99.906CERT

                               MFS SERIES TRUST XI

         CERTIFICATION PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT

    I, Maria F. Dwyer, certify that, to my knowledge:

    1. The Form N-CSR (the "Report") of MFS Series Trust XI (the "Registrant")
       fully complies for the period covered by the Report with the requirements
       of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

    2. The information contained in the Report fairly presents, in all material
       respects, the financial condition and results of operations of the
       Registrant.

Date: November 23, 2005               MARIA F. DWYER
      -----------------               -----------------------------------
                                      Maria F. Dwyer
                                      President (Principal Executive Officer)


A SIGNED ORIGINAL OF THIS WRITTEN STATEMENT REQUIRED BY SECTION 906 HAS BEEN
PROVIDED TO THE REGISTRANT AND WILL BE RETAINED BY THE REGISTRANT AND FURNISHED
TO THE SECURITIES AND EXCHANGE COMMISSION OR ITS STAFF UPON REQUEST.