Finasteride (Proscar--an orally active 5 alpha-reductase enzyme inhibitor) blocks the conversion of testosterone to dihydrotestosterone. The effects of finasteride in patients with benign prostatic hyperplasia were investigated in 2 double-blind, placebo-controlled studies. In study 1, 86 patients were treated with placebo or finasteride (5 to 80 mg. per day) for 12 weeks, followed by a 12-week drug-free period. After 12 weeks of treatment all doses of finasteride showed significant decreases in prostate volume. However, 12 weeks after discontinuation of finasteride prostate volume returned to near baseline values. In study 2, 104 patients were treated with placebo or finasteride (0.2 to 40 mg. per day) for 24 weeks. After 24 weeks of finasteride treatment prostate volume showed a mean decrease of 24% and 28% (p less than 0.01) in the 1 and 5 mg. groups, respectively. Lower doses had a lesser effect on prostate shrinkage. Maximum urinary flow showed a mean increase of 3.7 cc per second when the 1 and 5 mg. groups were combined. Symptom improvement was observed in the 1 and 5 mg. groups, although this was not statistically different from the placebo group due to the small sample size.