In 651 patients mapping of the prostate by 6 systematic sextant ultrasonography guided biopsies was performed without major side effects using the automatic biopsy gun. The histological findings provided data on patients with normal and abnormal prostates as determined by digital rectal examination. Only 3 of 72 nonurological patients (4%) with normal prostate specific antigen (PSA) levels of less than 4 ng./ml. had prostate cancer. Of the 259 patients with a firm prostate on digital rectal examination 105 (41%) had prostate cancer. For those with a PSA level of less than 4 and 4 ng./ml. or greater the positive biopsy rates were 13% and 58%, respectively. Of 56 patients with clinical stage B or C disease and a PSA level of less than 4 ng./ml. 20 (36%) had prostate cancer, compared to 155 of 187 (83%) with a PSA level of 4 ng./ml. or greater. Transrectal ultrasound was not helpful in screening for prostate cancer due to the low positive biopsy rate for hypoechoic lesions. However, among 175 patients with clinical stage B or C disease transrectal ultrasound identified 157 (90%) with prostate cancer.