About one-fifth had bladder or bowel trouble, researchers found.
The study doesn't compare these men — who were 70 to 89 at the end of the study — to others who did not treat their cancers or to older men without the disease. At least one study suggests that half that age group has sexual problems even when healthy.
The study isn't a rigorous test of surgery and radiation, but it is the longest follow-up of some men who chose those treatments.
Since early prostate cancers usually don't prove fatal but there are no good ways to tell which ones really need treatment, men must be realistic about side effects they might suffer, said one study leader, Dr. David Penson of Vanderbilt University.
"They need to look at these findings and say, 'Oh my gosh, no matter what I choose, I'm going to have some quality-of-life effect and it's probably greater than my doctor is telling me,'" he said.
The study appears in Thursday's New England Journal of Medicine.
Prostate cancer is the most common cancer in American men. In the United States alone, there were about 240,000 new cases and 28,000 deaths from the disease last year. Radiation or surgery to remove the prostate are common treatments when the disease is confined to the gland.
Men usually live a long time after treatment — 14 years on average — so it's important to see how they fare, said another study leader, Vanderbilt's Dr. Matthew Resnick.
The study involved 1,655 men diagnosed in 1994 or 1995, when they were ages 55 to 74. About two-thirds of them had surgery and the rest, radiation. They were surveyed two, five and 15 years later. By that time, 569 had died.
Men who had surgery had more problems in the first few years after their treatments than those given radiation, but by the end of the study, there was no big difference.
After 15 years, 18 per cent of the surgery group and 9 per cent of the radiation group reported urinary incontinence, and 5 per cent of the surgery group and 16 per cent of the radiation group said they were bothered by bowel problems. But the differences between the two groups could have occurred by chance alone once researchers took other factors such as age and the size of the men's tumors into account.
Impotence was "near universal" at 15 years, the authors write — 94 per cent of the radiation group and 87 per cent of the surgery group. But the difference between the groups also was considered possibly due to chance. Also, less than half of men said they were bothered by their sexual problems.
"These men do get some help from pills like Viagra, Cialis, Levitra," but it may not be as much as they would like and most men would rather not need those pills, Penson said.
The National Cancer Institute paid for the study. Two authors have consulted for several makers of prostate cancer treatment drugs.
No study is perfect and this one has many limitations, said Dr. Timothy Wilson, urology chief at City of Hope, a cancer centre in Duarte, Calif. Men who are having problems are more likely to complete follow-up surveys because they're angry, so that could skew results, he noted.
Still, "it's a high percentage" with side effects, said Wilson, who has been a paid speaker for two makers of surgery equipment.
"There's no question we overtreat" many cases of early prostate cancer, yet the disease is still the second-leading cause of cancer deaths in men. "We need to better sort out who really needs treatment," he said.
New England Journal: http://www.nejm.org
Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP