Prostate-specific antigen (PSA) is the most important tumor marker for prostate cancer. However, the diagnostic limits of PSA have to be taken into consideration because PSA is also secreted by normal prostate tissue and, with benign prostatic hyperplasia, false positives are possible. Although there is a direct correlation between the serum PSA concentration and the clinical stage of the tumor, PSA is not sufficiently reliable to determine the stage of the disease on an individual basis. Low serum PSA concentrations (less than 20 ng/ml) in patients with previously untreated prostate cancer seem to be predictive for a negative bone scan. Serum PSA values also reflect the prognosis of the patient. With respect to monitoring patients after definitive therapy, PSA is a very sensitive tumor marker. However, in a small number of patients PSA-negative tumor recurrences occur.