Patients with previously untreated category T3 to T4 Mo or Ml prostatic cancer were allocated randomly to receive 250 mg. cyproterone acetate per day, a loading dose of 500 mg. medroxyprogesterone acetate intramuscularly 3 times weekly for 8 weeks followed by 100 mg. orally twice daily, or 1 mg. diethylstilbestrol 3 times daily in a phase III trial (protocol 30761) performed by the genitourinary tract cooperative group of the European Organization for Research on the Treatment of Cancer. Of 236 patients entered 210 were eligible: 75 received cyproterone acetate, 71 medroxyprogesterone acetate and 64 diethylstilbestrol. Local and distant tumor response, time to progression, survival and toxicity were assessed. Patients treated with medroxyprogesterone acetate had a less favorable course with a shorter duration of survival and time to progression than those treated with the other 2 drugs. There was no significant difference between diethylstilbestrol and cyproterone acetate. Cardiovascular side effects were reported more often in patients treated with diethylstilbestrol than in those treated with cyproterone acetate but severe and lethal cardiovascular toxicity was relatively low in all groups. Other side effects were negligible. Further studies are required to establish the influence of effective hormonal treatment upon survival.