This study was designed to determine the efficacy of the 5α‐reductase inhibitor finasteride (Proscar, MK‐906) in men with reduced urinary flow rates and symptoms of urinary outflow obstruction secondary to benign prostatic hyperplasia. Forty‐five men were randomized to one of three groups receiving either placebo, 1 mg/day or 5 mg/day tinasteride for the first 12 months of the study period. At the end of this period all men received 5 mg/day finasteride for a further 2 years. Efficacy was determined by measurement of prostate volume, maximum urinary flow rate, and symptom score using a modified Boyarsky assessment. Prostate volume reduced by 20 and 27%. respectively, for those on 1 and 5 mg after the first year. At 3 years the volume had reduced by 43%. This reduction in prostate volume was associated with an improvement in maximum urinary flow rate by 50% (1 mg), and 35% (5 mg) at 1 year, and 36% at 3 years. The total, obstructive and non‐obstructive symptom scores decreased (improved) for patients on 1 and 5 mg tinasteride, with the total score reducing by 33% from baseline at year 3. The results demonstrate that finasteride causes a modest but signiticant clinical improvement in men with urinary outflow obstruction secondary to benign prostatic hyp