Casodex, a new nonsteroidal antiandrogen, is presently being investigated in phase III studies as a new hormonal treatment for advanced prostatic cancer. The safety and efficacy of Casodex at a dosage of 50 mg. daily for 24 weeks in patients with benign prostatic hypertrophy were investigated in this double-blind placebo-controlled trial, initially planned for 60 patients. Inclusion was discontinued at 30 patients after report of liver toxicity when Casodex was given long-term at high doses to mice. Prostate volume was decreased by 26.4% at the end of therapy (3.7% in the placebo group). The differences between Casodex and placebo therapy for the changes in values from baseline to 24 weeks of treatment in maximum urinary flow rate at spontaneous micturition and after instillation of saline were 0.8 and 1.4 ml. per second (not statistically significant). Pressure-flow examinations and frequency-volume charts did not show any treatment effect. Casodex patients tended to obtain more improvement in symptom scores than placebo patients, reaching statistical significance for irritative symptoms at week 24. Even if all patients had side effects, mostly mild, Casodex was well tolerated and patient compliance was excellent.