The clinical manifestations of benign prostatic hyperplasia (BPH) are related primarily to bladder outlet obstruction resulting from enlargement of the prostate gland. Transurethral prostatectomy is the most common treatment currently offered for BPH in the United States. The primary objective of the present randomized placebo controlled multicenter study was to determine the efficacy and safety of terazosin, a selective long-acting alpha 1-blocker, for the treatment of symptomatic BPH. A total of 285 men with symptomatic BPH was randomly assigned in equal proportions to receive placebo, or 2, 5 or 10 mg. terazosin administered once daily. Of the patients 237 completed the 4-week single-blind placebo lead-in and 12-week double-blind treatment periods. The primary outcome parameters were changes in peak and mean urinary flow rates, and changes in the Boyarsky symptom scores. All terazosin treatment groups exhibited significantly greater decreases in total Boyarsky symptom score than the placebo group. The 10 mg. terazosin group exhibited significantly greater increases in peak and mean urinary flow rates than the placebo group. The improvements in symptom scores and urinary flow rates did not reach a plateau within the dose range evaluated, suggesting that further efficacy may be achieved with doses of terazosin exceeding 10 mg. This study unequivocally demonstrates the safety and efficacy of terazosin for the treatment of BPH. Selective alpha 1-blockade is likely to gain widespread acceptance for the treatment of BPH due to its safety and efficacy.