<Page>

                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549

                ------------------------------------------------

                                   FORM 8-K/A
                                (Amendment No. 1)

                                 CURRENT REPORT
                     PURSUANT TO SECTION 13 OR 15 (d) OF THE
                       SECURITIES AND EXCHANGE ACT OF 1934

                DATE OF REPORT (DATE OF EARLIEST EVENT REPORTED)

                                DECEMBER 13, 2001

                ------------------------------------------------

                            NORTHWESTERN CORPORATION
               (Exact name of registrant as specified in charter)

                                    DELAWARE
                                    --------
                 (State or other jurisdiction of incorporation)


                0-692                              46-0172280
               -------                             ----------
      (Commission File Number)        (IRS Employer Identification Number)

                 125 SOUTH DAKOTA AVENUE, SIOUX FALLS, SD 57104
                 ----------------------------------------------
              (Address of registrant's principal executive office)

       Registrant's telephone number, including area code: (605) 978-2908
                                                           --------------

                                 Not Applicable
            -------------------------------------------------------
          (Former Name or Former Address, if Changed Since Last Report



<Page>



      The sole purpose of this Amendment No. 1 to our Current Report on Form 8-K
filed with the Securities and Exchange Commission (the "SEC") on December 12,
2001 is to file Exhibit 10.1, Exhibit 10.2, Exhibit 10.3, Exhibit 10.4,
Exhibit 10.5, Exhibit 10.6, Exhibit 10.7, Exhibit 10.8 and Exhibit 23.

ITEM 7. FINANCIAL STATEMENTS, PRO FORMA FINANCIAL INFORMATION AND EXHIBITS

(c)   EXHIBITS

EXHIBIT
NUMBER            TITLE
- -------           -----

10.1              Comprehensive Employment Agreement and Investment Program for
                  Merle D. Lewis, dated as of June 1, 2000.

10.2              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Richard R. Hylland, dated as of
                  March 1, 2001.

10.3              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Daniel K. Newell, dated as of
                  March 1, 2001.

10.4              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Michael J. Hanson, dated as of
                  March 1, 2001.

10.5              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Walter A. Bradley, III, dated as of
                  March 1, 2001.

10.6              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Kipp D. Orme, dated as of
                  March 1, 2001.

10.7              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Eric R. Jacobsen, dated as of
                  March 1, 2001.

10.8              Supplemental Income Security Plan for Directors, Officers and
                  Managers, as amended and restated effective July 1, 1999.

23                Consent of PricewaterhouseCoopers LLC.

99.1 *            Unaudited Combined Financial Statements of the Utility as of
                  and for the nine months ended September 30, 2001 and Audited
                  Combined Financial Statements of the Utility as of December
                  31, 2000 and 1999 and for each of the years in the three-year
                  period ended December 31, 2000.

99.2 *            Unaudited Pro Forma Combined Financial Information of
                  NorthWestern Corporation as of and for the nine months ended
                  September 30, 2001 and for the year ended December 31, 2000.

99.3 *            Unaudited Pro Forma Combined Financial Data of the Utility as
                  of and for the nine months ended September 30, 2001 and for
                  the year ended December 30, 2000.

* Previously filed.



<Page>




                                   SIGNATURES

Pursuant to the requirements of the Securities and Exchange Act of 1934, the
Registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized.

                                    NorthWestern Corp.
                                    (Registrant)

                                    By:   /s/ Kipp D. Orme
                                          ------------------------------
                                          Kipp D. Orme
                                          Vice President--Finance and Chief
                                                Executive Officer


December 13, 2001




<Page>



                                  EXHIBIT INDEX

EXHIBIT
NUMBER            TITLE
- -------           -----


10.1              Comprehensive Employment Agreement and Investment Program for
                  Merle D. Lewis, dated as of June 1, 2000.

10.2              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Richard R. Hylland, dated as of
                  March 1, 2001.

10.3              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Daniel K. Newell, dated as of
                  March 1, 2001.

10.4              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Michael J. Hanson, dated as of
                  March 1, 2001.

10.5              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Walter A. Bradley, III, dated as of
                  March 1, 2001.

10.6              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Kipp D. Orme, dated as of
                  March 1, 2001.

10.7              Comprehensive Employment Agreement and Equity Plan
                  Participation Program for Eric R. Jacobsen, dated as of
                  March 1, 2001.

10.8              Supplemental Income Security Plan for Directors, Officers and
                  Managers, as amended and restated effective July 1, 1999.

23                Consent of PricewaterhouseCoopers LLC.

99.1 *            Unaudited Combined Financial Statements of the Utility as of
                  and for the nine months ended September 30, 2001 and Audited
                  Combined Financial Statements of the Utility as of December
                  31, 2000 and 1999 and for each of the years in the three-year
                  period ended December 31, 2000.

99.2 *            Unaudited Pro Forma Combined Financial Information of
                  NorthWestern Corporation as of and for the nine months ended
                  September 30, 2001 and for the year ended December 31, 2000.

99.3 *            Unaudited Pro Forma Combined Financial Data of the Utility as
                  of and for the nine months ended September 30, 2001 and for
                  the year ended December 30, 2000.

*     Previously filed.