After lung cancer, prostate cancer is the leading cause of deaths from cancer among men in the United States. It will claim 40,000 lives in 1995.¹ Studies in the early 1990s demonstrated that levels of prostate-specific antigen (PSA), a serine protease, are elevated in most men with clinically important prostate cancer and that measuring them is the best means for early detection of the disease.²–⁵ In 1993, the American Cancer Society recommended that clinicians measure PSA in all men 50 years of age and older as part of an annual prostate examination and that PSA screening should begin at . . .