Urinary retention that necessitates catheterization after herniorrhaphy is a well known, but usually ignored, situation. Increased sympathetic activity resulting from surgery may be the contributing factor. Blockade of ct receptors in the bladder neck and urethral sphinchter may prevent postoperative urinary retention. In this prospective placebo-controlled study, the efficacy of prazosin in preventing postoperative urinary retention after herniorrhaphy was investigated in 156 patients. Patients were randomized into two groups. Patients in Group I (control) were given placebo orally 12 hours before surgery, just before surgery, and 12 and 24 hours after surgery. In Group II, 1 mg of prazosin was given in the same manner of placebo. Nine of 84 patients (10.8%) in the prazosin group and 18 of 72 patients (25%) in the placebo group developed urinary retention. Catheterization was required in only 3 patients (3.5%) in the prazosin group compared to 10 patients (13.8%) in placebo-treated group (P < 0.05). In conclusion, prophylactic use of prazosin after herniorrhaphy significantly reduced the incidence of urinary retention and catheterization.