The study finds that PSA blood tests every four years seem to cut the risk of death from prostate cancer. But it also finds that screening makes no difference in overall mortality rates because most men die of other causes — not their prostate tumors.

The results underscore a frustrating truth about this disease: most prostate cancers grow too slowly to threaten a man's life and there is no good way to tell which ones will. Finding cancer often leads to treatments that can cause impotence, incontinence and other problems. The PSA test also is just a measure of inflammation, which can be due to many things besides cancer.

"A man needs to make a choice for himself, realizing the benefits exist in theory, but the harms have been shown in every study that we've ever done in prostate cancer," said Dr. Otis Brawley, chief medical officer of the American Cancer Society. "If there is an overall mortality benefit from prostate screening it is very, very small."