Prostate-specific antigen (PSA) is the best single test for the early diagnosis of prostate cancer. Numerous studies have evaluated serum PSA in early detection protocols, and the Food and Drug Administration recently has approved the use of serum PSA testing, combined with digital rectal examination (DRE), as an aid in the detection of prostate cancer in men age 50 years and older. Much of the medical community's enthusiasm for PSA testing is derived from the use of total serum PSA measurements. Recent experience, however, suggests that the use of certain PSA transformations (density, slope, age-specific adjustments, and determination of unbound serum levels) may increase further the clinical use of this marker, making it an even better tool for clinicians who are seeking to diagnose early-stage prostate cancer. In this report, we review the current status of PSA testing in the United States.