The efficacy and safety profiles of finasteride were tested in 1,645 patients with benign prostatic hyperplasia (BPH) over a 12-month period. The following effects were observed: (1) a 60-80% reduction in serum dihydrotestosterone levels; (2) a 20% reduction in prostate volume; (3) significant increases in the maximum urinary flow rate compared with placebo; and (4) significant improvement in urinary symptoms, particularly obstructive symptoms. All effects were well maintained for the entire duration of the study. Finasteride had a good safety profile and was well tolerated.