BACKGROUND: Caudal administration of local anesthetics, opioids, and the alpha2 adrenergic agonists is effective for postoperative pain. Intrathecal and epidural opioids may commonly result in urinary retention. The purpose of this study was to evaluate the effects of fentanyl or clonidine on postoperative analgesia and urinary retention. METHODS: Forty patients undergoing a hemorrhoidectomy were randomly assigned to receive 20ml of 2% lidocaine and 1 : 200,000 epinephrine plus clonidine 100 microgram (group C100, n = 10), clonidine 150 microgram (group C150, n = 10), fentanyl 50 microgram (group F50, n = 10), or fentanyl 100 microgram (group F100, n = 10) for caudal anesthesia. Mean arterial pressure (MAP) and heart rate (HR) were measured before anesthesia and immediately postanesthesia, and every 5 min after caudal administration for 30 min. The following variables were recorded: onset time, analgesic time, voiding time, and urinary retention. RESULTS: The MAP at 20, 25, and 30 min in group C150 (91 +/- 7, 91 +/- 6, and 90 +/- 7 mmHg, respectively) was less than in group F50 (99 +/- 4, 101 +/- 6, and 101 +/- 5 mmHg, respectively) (P < 0.05). There was no difference in HR and onset time of anesthesia among the groups. Analgesic times in groups C150, F50, and F100 (270 +/- 22, 265 +/- 23, and 323 +/- 82 min, respectively) were longer than in group C100 (207 +/- 59 min), (P < 0.05). The number of patients using a urinary catheter to void was much higher in group F100 (7 patients) than in the groups C100, C150, or F50 (4, 5, and 3 patients, respectively) (P < 0.05). The voiding time was longer in group C150 than in group C100 (369 +/- 122 min vs 266 +/- 83 min, P < 0.05). No side effects were reported. CONCLUSIONS: We conclude that clonidine 150 microgram is adequate to provide effective analgesia and a low incidence of urinary retention during caudal anesthesia.