This randomized, placebo-controlled, double-blind study was performed to evaluate the efficacy and safety of once-a-day terazosin (10 mg/day) in ambulatory patients (n = 57) with benign prostatic hyperplasia (BPH). After a 4-week placebo lead-in and a 24-week treatment period with terazosin (both single-blind), 30 patients who responded to terazosin were randomly assigned to either the terazosin or placebo treatment group for 12 weeks. During the single-blind treatment period, the peak urine flow rate increased 54% from a baseline average of 7.76 ml/sec to 11.92 ml/sec after terazosin; the mean flow rate increased 55% from a baseline of 4.90 ml/sec to 7.59 ml/sec; and the residual volume decreased 56% from 93.1 ml to 40.7 ml. The mean obstructive symptom score, irritative symptom score and physician's global assessment score improved by 68%, 34% and 27%, respectively. All these changes were significant (P less than 0.05) when compared to baseline values. During the double-blind period, the improvement in all the variables was sustained in the terazosin group but not in the placebo group. Peak and mean urinary flow rates, and physician's global assessment showed significant (P less than or equal to 0.05) differences at the end of the double-blind period. Adverse events occurred only during the single-blind period. The most frequent were headache (n = 6), asthenia (n = 3), and hypotension (n = 3). In summary, terazosin administered once-a-day improved the obstructive and irritative symptoms of BPH, urine flow rates and residual volume. Terazosin was well tolerated.