The effects of finasteride, a potent 5 alpha-reductase inhibitor, were assessed in patients with benign prostatic hyperplasia. Patients were treated with finasteride or placebo for 24 weeks in a double-blind multicenter study followed by a 12-month open-extension period. After 24 weeks, finasteride-treated patients, when compared to placebo-treated patients, showed a significant reduction in prostate volume (22.5% median decrease) and prostate significant antigen (32.4% median decrease), a significant increase in maximum urinary flow (1.6 ml/s mean increase from baseline) and a significant improvement in their obstructive symptom scores (two-point decrease from baseline). Finasteride was well tolerated, and the improvements in prostate volume, maximum urinary flow rate and obstructive symptom scores observed in the controlled study were maintained throughout the extension study.