Prostatic specific antigen (PSA) can be detected in normal and benign hypertrophic prostates, as well as in prostatic cancer and its metastases. Since it appears in the serum, this glycoprotein has become an established marker for the detection and monitoring of prostate cancer. Using a radioimmunoassay (CIS--Biointernational, France), we found serum PSA levels higher than 4 ng/ml in 55 of 58 patients with prostatic cancer. The concentrations were proportional to tumor stage: significantly higher in stages C and D than in stages A and B (p less than 0.002). In all 6 cases with occult prostatic carcinoma (stage A), levels were higher than 15 ng/ml. PSA was found to be a good indicator of response to therapy, as well as a marker of tumor progression during follow-up. After radical prostatectomy serum PSA levels decreased to below 1 ng/ml. Following radiotherapy levels returned to normal within 1-6 months in 8 of 11 patients. In 21 of 23 with metastases serum PSA decreased during hormonal treatment. In 3 who responded initially to hormonal therapy, levels increased before clinical manifestation of tumor progression. We conclude that PSA is a sensitive serum marker for the diagnosis of prostatic cancer in cases of metastatic disease of unknown origin, as well as for monitoring the response to treatment of prostatic carcinoma. The use of PSA serum levels for screening for prostatic cancer is still controversial.