In a randomized phase III trial performed by the Urological Group of the European Organization for Research on Treatment of Cancer low dose estramustine phosphate (280 mg. twice daily for 8 weeks and 140 mg. twice daily thereafter) was compared to diethylstilbestrol (1 mg. 3 times daily) in patients with stages T3 to T4, M0 or M1 prostatic cancer. Of 248 patients entered 227 were evaluable for analysis: 115 received estramustine phosphate and 112 received diethylstilbestrol. The best response of the local tumor as assessed by palpation was seen in patients receiving diethylstilbestrol. There was no significant difference between treatments for response rate of metastases, interval to local progression, distant progression, over-all survival and death of carcinoma of the prostate. Duration of survival was correlated with the assessment of local response as determined by palpation. The response of distant lesions also was correlated closely with survival. Diethylstilbestrol (1 mg. 3 times daily) was associated with a significantly worse degree of cardiovascular toxicity than estramustine phosphate. This finding was especially obvious in patients who had no history of cardiovascular disease. Gastrointestinal toxicity occurred in 25 patients treated with estramustine phosphate, including 6 in whom cessation of treatment was necessary. Further studies are required to determine the optimum dose of diethylstilbestrol and estramustine phosphate, and to establish the best form of hormonal treatment for prostatic carcinoma.