Prostate cancer is the most frequently diagnosed cancer and the second-leading cause of cancer death among U. S. men. Men face an array of treatment options, including surgery, external beam radiation, brachytherapy (or radioactive seeds) and so-called watchful waiting.

"We think it's important for men to know the full range of outcomes," said the study's lead author, Janet Stanford of the Fred Hutchinson Cancer Research Center in Seattle. "Quite a substantial portion of men are impotent. Yet men in support groups say they were quoted a figure of 20% to 30% frequency,' extrapolating the track record of superstar surgeons. The study is published in Wednesday's Journal of the American Medical Association.

Past studies of side effects have come up with hugely differing rates. Impotence ranged from 29% to 75%, and incontinence from 4% to 40% in selected populations.

The new study, however, looked at a broad population of men treated at community medical centers across six states. Researchers also asked the volunteers --1,291 black, white and Hispanic men aged 39 to 79 years old -- to assess their own function, rather than relying on the doctor's report.

All this suggests the new study better reflects the true incidence of side effects in the population as a whole, said Ian Thompson, chief of urology at the University of Texas Health Science Center in San Antonio.