Hamm, who was a member of the U.S. team that won a silver medal in Athens four years ago, has been suffering from an ankle injury.

"I have been dealing with this for the last year and it recently has gotten worse here in Beijing," Hamm said. "Right now I am unable to perform my tumbling skills at the level that I need to. This has an impact on my ability to contribute to the team's goals and I believe by continuing I would be putting myself at further risk."

Paul Hamm, who won the all-around gold medal in Athens, suffered a broken bone in his right hand at a competition in May.

It wasn't immediately determined who would take Morgan Hamm's place on the team.

Qualifying in the men's events will be one of the highlights of the first full day of Olympic competition Saturday.