Prostate cancer is the most commonly diagnosed non–skin cancer among U.S. men. It can be life-threatening, and many men have cancer without knowing it. For those reasons, doctors sometimes look for prostate cancer in healthy men (screen for cancer) by measuring blood levels of prostate-specific antigen (PSA), a protein secreted by the prostate gland. High PSA levels can be caused by cancer and may lead a doctor to take a sample of prostate tissue to see whether cancer is present (biopsy). Most prostate cancer grows very slowly, however, and many men with prostate cancer die of other causes. Neither PSA testing nor prostate biopsy tells doctors with certainty which cases of prostate cancer are threatening and which require treatment. As a result, many men with slow-growing cancer have biopsies and treatment after PSA testing that they would not have needed if doctors had never tested. For that reason, a group of experts recently recommended against prostate cancer screening with PSA testing. They concluded that men are hurt more than they are helped by the test. Their recommendation was based on standard practice, where men are tested every year and referred for biopsy and treatment at certain PSA levels. It is possible that using the PSA test differently (for example, by testing less often) would still be useful but reduce the harms of unnecessary treatment that come from more frequent testing.