A series of 93 normotensive patients with benign prostatic hyperplasia and maximum urinary flow rates < 15 ml/s, treated at 2 hospital centres using an identical protocol, was randomly assigned to receive a 12-week course of treatment with prazosin or placebo in a double-blind parallel group trial. A total of 75 patients completed the study and were suitable for the final analysis. Prazosin was administered orally in doses of 0.5 mg and then 1 mg twice daily for 4 days and 2 mg twice daily for the remainder of the trial. Patients on treatment with prazosin exhibited a significantly increased maximum urinary flow rate as compared with placebo, with a significant reduction in maximum voiding detrusor pressure. Prazosin therapy did not produce a significant effect on either frequency or standard parameters of detrusor instability. A double-blind overall assessment of drug efficacy and tolerance significantly favoured prazosin therapy. A total of 30 patients receiving prazosin and 28 receiving placebo reported varied adverse effects. Eighteen patients were excluded from the final analysis, 10 being withdrawn because of adverse effects, 7 on treatment with prazosin and 3 in the placebo group. In long-term usage oral prazosin was well tolerated and appeared to improve obstructed voiding in patients with benign prostatic hyperplasia.