In two large, multicenter, double-blind, parallel, US phase III clinical trials, men with benign prostatic hyperplasia were randomized to receive tamsulosin, either 0.4 or 0.8 mg daily, or placebo for 13 weeks. Efficacy was determined by changes from baseline in American Urological Association (AUA) symptom scores and peak urinary flow and by percentages of responders with clinically meaningful (> 25%) AUA score improvement and at least 30% increase in peak urinary flow. Secondary efficacy parameters were AUA and Boyarsky irritative, obstructive, and individual scores; investigators’ global assessment; and a total quality-of-life evaluation. Also analyzed were laboratory test results and adverse events, including orthostatic and antihypertensive effects. A trend toward statistically significant improvement occurred in all primary and secondary efficacy endpoints at both dosages versus placebo, except for peak urinary flow rate at endpoint in one trial with 0.4 mg of tamsulosin (P = .064). Urinary flow rates increased within hours after first tamsulosin dose. No clinically or statistically meaningful sitting blood pressure or symptomatic orthostatic changes were seen, and no physical findings or alterations in laboratory or electrocardiographic results were attributable to treatment. Tamsulosin 0.4 and 0.8 mg daily had a rapid onset of action and was effective and well tolerated, with minimal differences observed between dosage groups. The incidence of side effects was similar to that with placebo, and efficacy was sustained with 0.4 mg daily.