OBJECTIVE Radical (three-quadrant) hemorrhoidectomy is a major anorectal surgery that may necessitate aggressive pain management. This study was undertaken to determine whether intravenous nalbuphine infusion as an adjuvant to epidural morphine could offer not only a good quality of pain relief but also a lower incidence of side effects. METHODS Sixty patients requiring epidural anesthesia for radical hemorrhoidectomy were enrolled in a randomized, double-blind study. At the end of the surgery, all patients received epidural morphine 4 mg for relief of postoperative pain. Thereafter, 2 mg and 3 mg of morphine were administered via the epidural route at 8 p.m. and 8 a.m., respectively, for a 48-hour observation period. Patients in group 1 received an adjuvant intravenous infusion of nalbuphine 15 micrograms/kg/h, whereas patients in group 2 received intravenous saline only. A rescue analgesic of intramuscular meperidine 40 mg (every 4 hours) was available for each patient. RESULTS All patients had adequate postoperative pain relief. Cumulative (48-hour) analgesic requirements were similar. During the 48-hour observation period, one patient in group 1 and six in group 2 demonstrated a PaCO2 above 45 mm Hg. No patient had an SaO2 below 90%. The incidence of nausea and/or vomiting was 13% in group 1 and 62% in group 2. The incidence of pruritus was 7% in group 1 and 62% in group 2. CONCLUSIONS The results suggest that intravenous nalbuphine infusion as an adjuvant for epidural morphine reduces the incidence of side effects without decreasing the quality of pain relief.