A study was done on 21 patients who had advanced carcinoma of the prostate (stage D) treated with 1 mg./kg. cis-diamminedichloroplatinum per week for 6 weeks. The infusions were then spaced every 3 weeks thereafter. One patient had never been treated previously and 20 patients were failures of previous therapy with estrogens and/or radiotherapy and/or chemotherapy. A partial objective clinical remission was seen in 9 of the 21 patients (43 per cent). This response lasted from 3 to 14 months, with an average of 5.8 months. The responses were evidenced by a 50 per cent or more decrease of lesions in the liver (2 patients), recalcification of a bone lytic lesion (1 patient), disappearance of positive lymph nodes in the neck (2 patients), disappearance of pleural effusion (1 patient), disappearance of lymphatic block of lower extremities (2 patients) and disappearance of lung metastases and ureteral obstruction (1 patient). Six patients (28.5 per cent) had a complete disappearance of the bone pain and became ambulatory and asymptomatic, 2 patients (9.5 per cent) with bony metastases remained asymptomatic and apparently stable and 4 patients did not respond to treatment and showed progression. Cis-diamminedichloroplatinum seems to be the most effective drug available to date for the treatment of advanced carcinoma of the prostate.