Prostatic blood flow was measured with 15oxygen-water by positron emission tomography using a 1-compartment model. A dynamic study method was applied to 9 normal subjects, 6 with benign prostatic hypertrophy (BPH) and 11 with advanced stages C to D2 prostatic adenocarcinoma. Prostatic blood flow was 15.7 +/- 7.5 ml. per minute per 100 gm. in normal controls, 17.7 +/- 5.2 ml. per minute per 100 gm. in BPH patients and 29.4 +/- 7.8 ml. per minute per 100 gm. in prostatic cancer patients. Prostatic blood flow negatively correlated well with age in the normal subjects. Prostatic blood volume was also estimated by the steady state method using 15oxygen-carbon monoxide. Prostatic blood volume was 8.1 +/- 2.6% in normal controls, 8.9 +/- 1.1% in BPH patients and 6.1 +/- 2.1% in prostatic cancer patients. Blood flow in the prostatic cancer tissue was higher than that in the normal (p < 0.001) or BPH (p < 0.01) tissue. A significant difference in prostatic blood volume was also observed between BPH and cancer tissues (p < 0.02).