The studies are continuing and may eventually provide more definitive answers, researchers said.
In the meantime, experts say men should do what most guidelines advise: Talk to your doctor about the pros and cons of having the tests before deciding whether it is the right choice for you.
The studies were released Wednesday by the New England Journal of Medicine in connection with a conference in Sweden.
"The hope was there that there'd be a clear answer. Either that there was so little or no benefit that it clearly wasn't worth the risks. Or that the benefit was so large, that it was," said Dr.
Michael Barry of Massachusetts General Hospital, who wrote an accompanying editorial in the journal. "What we're left with is something in between - that the benefit is fairly small ... and the risks are pretty big."
Prostate cancer is the most common cancer in men in the U.S. and Europe. More than 186,000 cases will be diagnosed this year in American men and 28,660 will die of the disease.
Screening is done with a blood test that measures prostate specific antigen, or PSA. Levels of PSA can be high for many reasons and a biopsy is needed to confirm a tumor. Many tumors grow so slowly that they won't be a threat, but there's no sure way to tell which are the dangerous ones.
And there's no agreement on the best treatment approach - "watchful waiting," surgery, hormone therapy or radiation. The treatments can lead to impotence and incontinence.
No major medical group recommends routine screening because there's no proof that it actually saves lives. That's the question the teams of researchers took on when the studies began in the 1990s.
Both groups are reporting their results a few years early. In the U.S., a panel monitoring the research decided there was enough evidence so far that screening wasn't saving lives and may have been leading to unnecessary treatment with serious side effects.
That study involved 76,693 men ages 55 to 74 from 10 cities.
They were assigned to get six annual PSA tests and four digital rectal exams or regular care from their doctors, which could include screening. About half of the men getting usual care ended up getting screened at some point.
After seven to 10 years of follow-up, more cases of prostate cancer were found in the group that got annual screening, but no difference between the groups in the number of cancer deaths. The researchers plan to follow all the men for at least 13 years.
The U.S. researchers cautioned against comparing their results to the European research. Differences in how the studies were done and improvements in treatment may have contributed to the different results, they said.
The European results were based on 162,243 men between 55 and 69 in seven countries. Generally, the men were offered screening every four years or they got none. On average, they were followed for about nine years.
They found that screening lowered the rate of death by 20 percent.
Dr. Fritz Schroder, lead author of the study from the Netherlands' Erasmus Medical Center in Rotterdam, said he believes that the study will show even more lives saved as the men are followed years longer.
The European researchers noted a high risk of overdiagnosis - the finding and treating of cancers that wouldn't threaten lives.
They said 48 additional men would need to be treated to prevent one death from prostate cancer.
"My interpretation of the two studies together is that PSA screening likely does save some lives but does significant more harm," said Dr. Otis Brawley, the American Cancer Society's chief medical officer.
The researchers and others agreed the results bolster advice that screening shouldn't be done in those with a limited life-expectancy. U.S. guidelines issued last year said men over 75 shouldn't be screened; no recommendation was made for younger men but they were advised to discuss the test with their doctor.
What's needed, the researchers said, is a better way to tell which prostate cancers need to be treated.
"When we find prostate cancer, we don't know if it is a killer cancer or what has been termed a toothless lion - the kind of cancer that men will die with, not of," said the U.S. study's leader, Dr. Gerald Andriole of Washington University School of Medicine in St. Louis. He said the tendency is to treat all patients aggressively.
The U.S. study is part of a National Cancer Institute project; the European study was supported mostly by government and health agencies. Some of the researchers report receiving grants and fees from drugmakers; one holds a patent for a PSA test.
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