Prostate-specific antigen (PSA) is a kallikrein-like serine protease that is secreted exclusively by the epithelial cells of all types of prostatic tissue, benign and malignant. Its serum concentration is raised in men with prostatic disease including cancer. We have evaluated its usefulness in the diagnosis of prostate cancer by measuring serum PSA concentrations in 260 men aged 50 years or over. All had abnormalities at digital rectal examination (DRE) involving suspected cancer, signs and symptoms of benign prostatic hyperplasia and equivocal findings on DRE, and miscellaneous other conditions, including hematospermia, chronic prostatitis and microscopic hematuria. Transrectal prostatic needle biopsies were performed in the men with abnormal findings on DRE or elevated serum PSA (above 4 ng/ml). Serum PSA ranged from 4.0 to 9.9 ng/ml in 14 (5%) of the 260 men. Four of the men in this group (31%) who underwent prostatic biopsy had prostate cancer. Serum PSA levels greater than or equal to 10.0 ng/ml were found in 8 (3%) of the 260 men. 5 of these 8 (63%) who underwent prostatic biopsy had cancer. If DRE alone had been used to screen the men having biopsies, 4 of the 10 cancers (40%) would have been missed. If PSA alone had been used to screen these men, only 1 of the 10 cancers would have been missed. Serum PSA measurement was more reliable than DRE for detecting prostate cancer. Since these two methods do not always detect the same malignant tumor, the combined use of DRE and PSA testing affords a more complete evaluation of the prostate gland for malignant involvement.