As the P.S.A. test has grown in popularity, the devastating consequences of the biopsies and treatments that often flow from the test have become increasingly apparent. From 1986 through 2005, one million men received surgery, radiation therapy or both who would not have been treated without a P.S.A. test, according to the task force. Among them, at least 5,000 died soon after surgery and 10,000 to 70,000 suffered serious complications. Half had persistent blood in their semen, and 200,000 to 300,000 suffered impotence, incontinence or both. As a result of these complications, Richard J. Ablin, who in 1970 discovered a prostate-specific antigen, has called its widespread use a “public health disaster.”

One in six men in the United States will eventually be found to have prostate cancer, making it the second most common form of cancer in men after skin cancer. An estimated 32,050 men died of prostate cancer last year and 217,730 men received the diagnosis. The disease is rare before age 50, and most deaths occur after age 75.

Not knowing what is going on with one’s prostate may be the best course, since few men live happily with the knowledge that one of their organs is cancerous. Autopsy studies show that a third of men ages 40 to 60 have prostate cancer, a share that grows to three-fourths after age 85.

P.S.A. testing is most common in men over 70, and it is in that group that it is the most dangerous since such men usually have cancerous prostate cells but benefit the least from surgery and radiation. Some doctors treat patients who have high P.S.A. levels with drugs that block male hormones, although there is no convincing evidence that these drugs are helpful in localized prostate cancer and they often result in impotence, breast enlargement and hot flashes.

Of the trials conducted to assess the value of P.S.A. testing, the two largest were conducted in Europe and the United States. Both “demonstrate that if any benefit does exist, it is very small after 10 years,” according to the task force’s draft recommendation statement.

The European trial had 182,000 men from seven countries who either got P.S.A. testing or did not. When measured across all of the men in the study, P.S.A. testing did not cut death rates in nine years of follow-up. But in men ages 55 to 69, there was a very slight improvement in mortality. The American trial, with 76,693 men, found that P.S.A. testing did not cut death rates after 10 years.

Dr. Eric Klein of the Cleveland Clinic, an expert in prostate cancer, said he disagreed with the task force’s recommendations. Citing the European trial, he said “I think there’s a substantial amount of evidence from randomized clinical trials that show that among younger men, under 65, screening saves lives.”