This randomized double-blind crossover trial was conducted to assess the effects of prazosin, an alpha 1-adrenoceptor blocking drug, on the voiding of 35 patients with benign prostatic obstruction. Maximum and mean flow rates, residual urine, blood pressure and heart rate were measured at baseline and 2, 4, 6, and 8 weeks after starting the treatment with placebo or prazosin. At 4 weeks the treatments were switched over. The patients filled micturition charts at home and scored their voiding associated feelings. The maximum and mean flow rates increased significantly during prazosin treatment, as also did the maximum and mean voided volumes. Residual urine decreased and voiding improved subjectively but these changes were not statistically significant. Blood pressure was lowered and heart rate increased. Prazosin caused postural dizziness more often than placebo. Prazosin seems to offer an alternative to improve voiding in some patients with prostatic obstruction.