A new study shows how important it is for men to carefully consider treatments for early stage prostate cancer. Fifteen years after surgery or radiation treatment, nearly all of the older men in the study had some problems having sex.

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.

Prostate cancer is the most common cancer in American men.

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 today’s New England Journal of Medicine.