The outcry among many physicians and patients over a government panel's recent announcement that healthy men should no longer receive P.S.A. blood testing to detect prostate cancer is rooted in a long and impassioned history among cancer screening advocates that early detection must always save lives. But as science has taught us, that’s not always the case. As early as 1913, physicians and laypeople formed the American Society for the Control of Cancer, which later became the American Cancer Society, bearing this hopeful message: “With early recognition and prompt treatment, the patient’s life may often be saved.” The idea had some scientific basis. “Delay kills!” posters bluntly warned. But research also emerged that questioned the cancer society’s original assumption that cancer was a local disease that spread in a gradual and orderly fashion. Scientists had found cancer cells in the blood of patients with seemingly tiny, localized cancers, suggesting that cancer cells could spread silently early in the course of disease. In that case, so-called early detection might not really be early, or of much value.