A pilot study was done using 19 patients with bladder cancer to test the efficacy of cis-diamminedichloroplatinum as a single agent to treat transitional cell carcinoma. Fourteen patients had stage D lesions, 3 patients had stage C and 2 patients had stage B tumors. The patients received 1 mg./kg. cis-diamminedichloroplatinum mixed with 2,000 cc dextrose 5 per cent in one-third normal saline solution, 37.5 gm. mannitol and 40 mEq. potassium chloride. The mixture was given in a slow infusion lasting 6 to 8 hours every week for the first 6 weeks and every 3 weeks thereafter. In the patients with stage D disease 1 complete clinical remission, 7 partial clinical remissions, 1 minimal objective response and 1 subjective response were observed. Four patients did not respond to the treatment. In the 3 stage C patients no response to the drug was observed. In the stage B patients 1 partial remission and 1 minimal objective response were seen. A total objective response (complete remission and partial remission) was seen in 9 of the 19 patients (47 per cent). The duration of the responses varied from 2 to 10 months, with an average of 4.8 months. These promising results seem to indicate the high degree of effectiveness of cis-diamminedichloroplatinum against transitional cell carcinoma of the bladder.