The European study saw no benefit of screening in the first seven years of follow-up.

Screening is not only an issue in prostate cancer. If the European study is correct, mammography has about the same benefit as the PSA test, said Dr. Michael B. Barry, a prostate cancer researcher at Massachusetts General Hospital who wrote an editorial accompanying the papers. But prostate cancers often are less dangerous than breast cancers, so screening and subsequent therapy can result in more harm. With mammography, about 10 women receive a diagnosis and needless treatment for breast cancer to prevent one death. With both cancers, researchers say they badly need a way to distinguish tumors that would be deadly without treatment from those that would not.

When the American and European studies began, in the early 1990s, PSA testing was well under way in the United States, and many expected that the screening test would make the prostate cancer death rate plummet by 50 percent or more. Dr. Brawley was at the cancer institute then, though not directly involved with its prostate cancer screening study. But he saw the reactions.

Some urologists said the study was unethical, because some people would not be screened, and demanded it be shut down, he said. One group of black urologists encouraged black men not to participate because blacks have a greater risk of prostate cancer and it seemed obvious they should be screened.

Some thought that they would see fewer cancer deaths among screened men as quickly as five years. But it became clear that screening would not have a large, immediate effect  if it did, the studies would have been stopped and victory declared. Cancer researchers began turning to less rigorous sources of data, with some arguing that screening was preventing cancer deaths and others arguing it was not.

In the United States, many men and their doctors have made up their minds  most men over age 50 have already been screened, and each year more than 180,000 receive a diagnosis of prostate cancer. In Europe, said Dr. Fritz H. Schröder of Erasmus University, the lead author of the European study, most men are not screened. “The mentality of Europeans is different,” he said, and screening is not so highly promoted.

Both studies will continue to follow the men. It remains possible that the United States study will eventually find that screening can reduce the prostate cancer death rate, researchers say, or that both studies will conclude that there is no real reduction.

“I certainly think there’s information here that’s food for thought,” Dr. Brawley said.

The benefits of prostate cancer screening, he said, are “modest at best and with a greater downside than any other cancer we screen for.”