John W. Sidgmore, a leading Internet entrepreneur who became chairman and chief executive of the scandal-ridden WorldCom last year when its founder, Bernard J. Ebbers, was forced to resign, died yesterday of kidney failure, according to a statement issued by his family. He was 52 and lived in Potomac, Md.

Mr. Sidgmore became a senior executive at WorldCom in 1996 when it swallowed [MFS Communications Company, Incorporated] in a $14 billion deal. That takeover occurred shortly after Mr. Sidgmore arranged to merge [UUNET Technologies, Incorporated], the Internet networking company he ran, with MFS.

Initially, he and Mr. Ebbers worked closely together, although the compact, rock-'n'-roll-loving Mr. Sidgmore, who was a fixture in Washington, and the towering Mr. Ebbers, who operated out of Clinton, Miss., made a study in contrasts.

Mr. Sidgmore had built UUNet into the dominant carrier of Internet traffic. He helped WorldCom maintain a huge share of that traffic even as demand exploded and numerous other carriers spent billions of dollars to compete. He also played a central role in WorldCom's 1999 takeover of MCI, which made it the second-largest long-distance phone company after AT&T.

That same year, though, Mr. Sidgmore had a falling-out with Mr. Ebbers that began when Mr. Ebbers killed a deal Mr. Sidgmore had negotiated to acquire Nextel, a wireless phone company. He played little role in day-to-day operations for more than two years until Mr. Ebbers was forced to resign in April 2002.

Mr. Sidgmore spent much of his time advising or helping to manage start-up technology companies in the Washington area.

Mr. Sidgmore's plans to rescue WorldCom were sidetracked as evidence mounted that the company had been involved in one of the largest accounting frauds in history. By late July, less than three months after Mr. Sidgmore took over, his task became leading the company into the largest bankruptcy reorganization ever.

Mr. Sidgmore tenaciously fought critics who advocated that WorldCom be broken up in bankruptcy. He maintained his wry sense of humor, which had helped make him one of the industry's most well-liked executives. In a statement yesterday, WorldCom said that he had had a profound positive effect on its employees.

Mr. Sidgmore was never charged with any misconduct, but as the scope of WorldCom's crisis widened, so did pressure from investors and regulators for a more thorough break with the past. Michael D. Capellas, the executive who led Compaq into its merger with Hewlett-Packard, succeeded him a year ago.

''He created the backbone of the Internet,'' said Frank Dzubeck, president of Communications Network Architects, a consulting firm in Washington. ''It was made up of islands until he put it together at UUNet to link the various service providers. It's unfortunate that he is going to be better remembered for the end of his career.''

John William Sidgmore was born April 9, 1951, in Suffern, N.Y. Most of his early business career was spent with General Electric Information Systems, where he rose through the management ranks before leaving in 1989 to take on the challenge of rescuing Intellicom Solutions, a troubled computer services company. The company was sold to the Computer Sciences Corporation in 1991.

Mr. Sidgmore's rise to prominence began when he joined UUNet in 1994 and focused it on providing Internet service to businesses. A year later, he took it public in one of 1995's most successful stock sales.

Mr. Sidgmore is survived by his wife, Randi, and his son, Michael, both of Potomac; his father, John; a brother, Michael; and a sister, Jean.

John Sidgmore, 52, the WorldCom executive who helped reveal the accounting troubles that led to the biggest bankruptcy filing in U.S. history, died Thursday of complications associated with acute pancreatitis, a family spokesman announced.

A resident of Potomac, Md., Sidgmore was named president of WorldCom in April 2002, when the firm’s founder, Bernard Ebbers, was forced to resign. Sidgmore quickly disclosed the company’s accounting problems. He resigned last December but continued to work with WorldCom as a consultant.

Sidgmore had been vice chairman of the board when the company was involved in an accounting shell game to make it appear more profitable, but he said in July 2002, soon after the bankruptcy declaration, “I haven’t been involved in the direct operations of the company in several years.”

Earlier that month, he had expressed optimism about the firm, yet also apologized for its “past transgressions” and vowed to cooperate as the government investigated.

“We want the bad guys exposed,” Sidgmore said at a news conference. “We want the bad guys punished.” The Securities and Exchange Commission had filed civil fraud charges a week earlier against WorldCom after the company disclosed that it had improperly accounted for nearly $4 billion in expenses, thus inflating its earnings. The news sent the company’s stock plummeting below a dime.

Sidgmore was born in Suffern, N.Y. He earned a bachelor’s degree in economics from the State University of New York in 1973 and spent most of his early business career with General Electric Information Systems.