8-K

QuickLinks -- Click here to rapidly navigate through this document

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, DC 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the
Securities Exchange Act of 1934

Date of report (Date of earliest event reported): April 20, 2004

WESTERN GAS RESOURCES, INC.
(Exact Name of Registrant as Specified in Charter)

Delaware (State of Other Jurisdiction of Incorporation)	1-10389 (Commission File Number)	84-1127613 (IRS Employer Identification No.)
1099 18th Street, Suite 1200, Denver, Colorado (Address of Principal Executive Offices)		80202 (Zip Code)

(303) 452-5603
(Registrant's telephone number, including area code)

(Former Name or Former Address, if Changed Since Last Report)

Item 5. Other Events

        On April 22, 2004, Western Gas Resources, Inc. issued a press release announcing the completion of its previously announced redemption of all remaining outstanding shares of its $2.625 Cumulative Convertible Preferred Stock.

        The press release is attached as an exhibit to this Form 8-K and incorporated herein by reference.

Item 7. Financial Statements and Exhibits

(c)

Exhibit:

99

Press release, dated April 22, 2004, announcing the completion of the previously announced redemption of all remaining shares of Western Gas Resources, Inc. $2.625 Cumulative Convertible Preferred Stock.

2

SIGNATURES

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

	WESTERN GAS RESOURCES, INC. (Registrant)
Date: April 22, 2004	By:	/s/  WILLIAM J. KRYSIAK Name: William J. Krysiak Title: Executive Vice President and Chief Financial Officer

3

EXHIBIT INDEX

Exhibit Number	Description
99	Press release, dated April 22, 2004, announcing the completion of the previously announced redemption of all remaining shares of Western Gas Resources, Inc. $2.625 Cumulative Convertible Preferred Stock.

4

QuickLinks

Item 5. Other Events
Item 7. Financial Statements and Exhibits

SIGNATURES
EXHIBIT INDEX

EX-99

QuickLinks -- Click here to rapidly navigate through this document

Exhibit 99

WESTERN GAS RESOURCES, INC.
ANNOUNCES COMPLETION OF REDEMPTION OF ALL REMAINING SHARES OF
$2.625 CUMULATIVE CONVERTIBLE PREFERRED STOCK
ON APRIL 20, 2004

DENVER, April 22, 2004. Western Gas Resources, Inc. (the "Company" or "Western") (NYSE:WGR) announced today the results of its previously announced redemption of all the shares of its $2.625 Cumulative Convertible Preferred Stock (the "Preferred Stock") completed on April 20, 2004. Of the remaining shares of Preferred Stock called for redemption, holders elected to convert 1,237,653 shares and Western redeemed all of the remaining 7,939 shares. Western issued approximately 1,556,791 shares of its common stock for the shares of Preferred Stock tendered for conversion and paid $391,000 for the shares of Preferred Stock redeemed. As of April 21, 2004, following the redemption, approximately 36,815,407 shares of Western's common stock are issued and outstanding.

Following completion of the redemption on April 20, 2004, there are no remaining shares of the Preferred Stock outstanding. Any holder of certificates representing the Preferred Stock that have not been surrendered for cancellation no longer have any rights pursuant to such certificates except for the right to receive the redemption price of $50.467 per share, without interest thereon, upon surrender of the certificates. The Company intends to apply to the New York Stock Exchange for delisting of the Preferred Stock.

Company Description.    Western is an independent natural gas explorer, producer, gatherer, processor, transporter and energy marketer providing a broad range of services to its customers from the wellhead to the sales delivery point. The Company's producing properties are located primarily in Wyoming, including the developing Powder River Basin coal bed methane play, where Western is a leading acreage holder and producer, and the rapidly growing Pinedale Anticline. The Company also designs, constructs, owns and operates natural gas gathering, processing and treating facilities in major gas-producing basins in the Rocky Mountain, Mid-Continent and West Texas regions of the United States. For additional Company information, visit Western's web site at www.westerngas.com.

Investor Contact: Ron Wirth, Director of Investor Relations
(800) 933-5603 or (303) 252-6090

e-mail: rwirth@westerngas.com

QuickLinks

WESTERN GAS RESOURCES, INC. ANNOUNCES COMPLETION OF REDEMPTION OF ALL REMAINING SHARES OF $2.625 CUMULATIVE CONVERTIBLE PREFERRED STOCK ON APRIL 20, 2004