Catalona wanted to catch these cancers early, when they might be curable. He noticed that men with more advanced cancers at the time of surgery tended to have the highest P.S.A. levels. Could there be a bright line, a “safe” level of P.S.A. that could distinguish healthy men from those with prostate cancer? After reviewing his own patient records, he decided the cutoff level should be 4 nanograms of P.S.A. per milliliter of blood. He followed up with a study of 1,653 patients. The results, published in 1991 in The New England Journal of Medicine, showed that P.S.A. testing could detect prostate cancer several years earlier than a digital rectal exam.

The test quickly gained powerful support: Gerald Murphy, who held the position at the American Cancer Society now held by Brawley, pushed the society to endorse the test. In 1996, Gen. H. Norman Schwarzkopf, a prostate-cancer survivor, appeared on the cover of Time magazine over the statement “There’s a simple blood test everyone should know about.”

By then, doctors were using the test for routine screening. “P.S.A. testing was so easy,” says H. Gilbert Welch, a professor of medicine at the Dartmouth Institute (full disclosure: one author of this article is an instructor at Dartmouth). Doctors were predisposed to use the test for several reasons. First and foremost, there was the perception that early detection could save lives. It was also easy to administer. “It was a blood test,” Welch says. “You didn’t need equipment. . . . You didn’t need to put any scopes up any part of the body. Heck, you didn’t even need to ask the patient if he wanted it; you could just check off the box on a list of tests, like cholesterol, when you did a blood draw.” Today it’s common for doctors to order the P.S.A. test and patients to take it without talking about what it might really mean.

At one time, Otis Brawley, too, assumed that routine screening was the best medical practice. Sitting in his living room in an Atlanta suburb, Brawley recounted his transformation from believer to skeptic. In 1988, after medical school at the University of Chicago, Brawley landed a prestigious fellowship at the National Cancer Institute in Bethesda, Md. There he came under the tutelage of Barnett Kramer, an oncologist and epidemiologist who went on to become the associate director of the institute’s early detection and community oncology program. Kramer walked Brawley through a short history of screening, beginning with the Pap smear, which has been an unqualified success, significantly cutting cervical-cancer deaths.

But other cancer screening tests had not worked out so well. For example, researchers at the Mayo Lung Project conducted a study between 1971 and 1983 to determine whether frequent chest X-rays could help reduce deaths from lung cancer. Chest X-rays detected lots of suspicious spots and shadows on the lungs and probably led to some cures of early lung cancers, but the study ultimately found no difference in death rates between the patients who were screened and those who were not. Kramer suggested one probable explanation: diagnosing the spots picked up by X-ray often requires surgery, which carries a small but definite risk. Brawley knew that many spots seen on X-rays are simply old scars or minor abnormalities commonly seen in healthy people. With so many innocent blips detected, complications from lung biopsies and other invasive tests, along with treatment complications, could kill enough patients to negate any benefit from early detection.

Prostate cancer is the second-leading cause of cancer death among men, after lung cancer. In 2009, it was diagnosed in approximately 192,000 men. A small number of tumors are very aggressive, but the majority of prostate tumors are not likely to cause death. They grow very slowly, and only a fraction break out of the prostate, seed new tumors in other parts of the body and kill the patient. The current thinking is that about 30 percent of men in their 40s have prostate cancer, 40 percent of men in their 50s and so on, right up to 70 percent of men in their 80s. Yet only 3 percent of all men die from the disease. In other words, far more men die with prostate cancer than from it, and only a tiny fraction of prostate cancers ever cause symptoms, much less death.

But here is the tricky part: Unless there are symptoms or a finding on a physical exam, doctors generally cannot accurately predict which cancers are destined to be indolent, to sit around for years growing slowly, if at all, and those that will ultimately prove lethal.