Little is known about the efficacy of flutamide monotherapy in previously untreated patients with prostatic carcinoma. In this study, 40 patients with advanced disease were treated with 250 mg flutamide, three times daily. The mean follow-up was 7 months. After 3 months, 35 patients were evaluable for efficacy; 17 showed a partial response and 18 showed no change. Tumor response after 6 months was evaluated in 22 patients; 10 had a partial response, nine had stable disease, and three had progression. The level of prostate-specific antigen was reduced markedly following 6 months' treatment with flutamide. Levels of testosterone increased slightly but significantly, and were still elevated not significant after a follow-up period of 1 year. Follicle-stimulating hormone did not change markedly, whereas luteinizing hormone rose significantly. Eighteen patients experienced mild gynecomastia and eight suffered diarrhea. In two patients, flutamide was discontinued for 2 weeks due to serious diarrhea. One patient was withdrawn after 6 weeks because of cholestatic hepatitis. Sexual potency was evaluated in 15 patients, 10 of whom remained sexually active during treatment. Flutamide monotherapy was concluded to be relatively safe and effective in patients with advanced prostatic cancer.