Bill Gates joined the board of Warren Buffett's Berkshire Hathaway investment company yesterday, formalizing the relationship between two bridge buddies who happen to be the richest men in the world.

Buffett is considered a major influence on Gates' business philosophy and spoke out against the U.S. antitrust case against Microsoft.

Now Gates has a chance to return the favor by helping run Berkshire. The vacancy on Berkshire's board was created when Buffett's wife, Susan, died of a stroke in July.

Gates is Microsoft's chairman and serves on the board of Icos, the Bothell biotech company. He was not available for an interview but issued a statement: "I am delighted and honored to have been asked to serve on the board of this very successful company."

Buffett is asking his friend to help choose his replacement.

"The primary job of our directors is to select my successor, either upon my death or disability, or when I begin to lose my marbles," Buffett wrote in a letter to shareholders sent in February.

Berkshire owns an assortment of companies, including Geico Direct Auto Insurance, Fruit of the Loom and Benjamin Moore paint.

One property, Dairy Queen, could pose a conflict of interest for Gates, a devoted patron of the Burgermaster next to Microsoft's early office in Bellevue.

Burgermaster used to be on the speed dial setting of Microsoft phones, and the drive-in built a stairway providing direct access. Manager Paul Tomicich isn't too worried about the Berkshire appointment. He said Gates still dines there about once a month.

"He's proclaimed Burgermaster to be his favorite burger over the years," Tomicich said.