BACKGROUND: Urinary retention is the most common and distressing complication in the postoperative period after an epidural morphine administration. In this prospective placebo-controlled study, the efficacy of doxazosin on preventing urinary retention after epidural morphine was investigated. METHODS: A total of 75 males who underwent elective arthroscopic knee surgery were randomized into three groups. Patients in group I received a placebo orally before surgery. Group II patients received 4 mg of doxazosin, and group III patients received 8 mg of doxazosin. In all patients, 2 mg of epidural morphine in 6 ml normal saline was administrated by the epidural route in the L3-4 interspace. General anesthesia was induced in all patients. The volumes of the first two postoperative urine voidings, the time intervals between the end of surgery and the first micturition and the need for bladder catheterization were recorded. RESULTS: There were statistically no significant differences among the groups in the first two postoperative voidings (Group I 449 ml/445 ml, Group II 343 ml/388 ml, Group III 376 ml/380 ml). No significant differences were noted among the three groups in the mean time between the end of the surgery and the first micturition (Group I 467 minutes, Group II 440 minutes, Group III 366 minutes). Also, with urinary catheterization, there were no differences among the groups (G I6/25, G II 5/25, G III 7/25). CONCLUSIONS: Prophylactic use of doxazosin does not prevent postoperative urinary retention after epidural morphine.