Fifty-eight normotensive patients with benign prostatic hyperplasia and maximum urinary flow rates of less than 15 ml/s were randomly assigned to receive a 12-week course of treatment with prazosin or placebo in a double-blind parallel group trial. Prazosin was administered orally in doses of 0.5 mg and then 1 mg twice daily for 4 days each and 2 mg twice daily for the remainder of the trial. Patients on treatment with prazosin had an increased urinary flow rate as compared to placebo with a significant reduction in maximum voiding detrusor pressure and maximum detrusor pressure at peak urinary flow. Although a significant effect on frequency was not demonstrated, standard parameters of detrusor instability were reduced. Investigators’ double-blind overall assessment of efficacy significantly favoured the prazosin treatment. Twelve patients were excluded from the final analysis, 8 being withdrawn because of adverse effects, 5 on treatment with prazosin and 3 in the placebo group. Oral prazosin appears to be safe and effective in the long-term treatment of patients with benign prostatic hyperplasia.