USA Today notes the doctors' exodus comes just six months before millions of baby boomers begin enrolling in the federal government healthcare program.
"Physicians are saying, 'I can't afford to keep losing money,'" said Lori Heim, president of the American Academy of Family Physicians.
AAFP reports 13 percent of doctors who responded to a survey said they didn't participate in Medicare last year, up from 8 percent in 2008 and 6 percent in 2004.
More doctors are refusing new Medicare patients in part because Congress failed to stop an automatic 21 percent cut in payments that took effect Friday, results of state and national surveys suggest.
The American Medical Association said 17 percent of more than 9,000 doctors surveyed said they restrict the number of Medicare cases, and the rate rises to 31 percent for primary care physicians.
Shortages of primary care physicians already alarm many experts, and the seniors group AARP says record numbers of doctors refusing Medicare will make matters worse.
The federal health insurance program paid on average 78 percent of what private insurers paid in 2008.
Some doctors in states including New York and North Carolina have left Medicare.
In New York state, Leah McCormack, the president of the New York Medical Society, is among 1,100 doctors who have left the program.
"I'm making a statement," said McCormack, a New York City dermatologist. "Many physicians are really being forced out of private practice."