BACKGROUND: Epidural morphine has been commonly used to provide postoperative pain relief, but it has many side effects such as pruritus, nausea, vomiting, and respiratory depression. The purpose of this study was to evaluate the analgesic efficacy and side effects of epidural morphine compared with epidural nalbuphine. METHODS: Fifty nine patients were randomly divided into 2 groups. For group M (n = 30), a bolus of 7 ml of saline and 2 mg of morphine were administered and for group N (n = 29), a bolus of 7 ml of saline and 4 mg of nalbuphine were administered. Continuous epidural analgesia was induced with morphine 4 mg, 0.5% bupivacaine 20 ml, 2% lidocaine 20 ml and normal saline 60 ml by a 2day infuser in group M, and with nalbuphine 20 mg, 0.5% bupivacaine 20 ml, 2% lidocaine 20 ml and normal saline 58 ml by a 2day infuser in group N. We compared the analgesic effect and side effects of the two groups for 48 hours. RESULTS: No significant hemodynamic changes were seen in any of the groups. The analgesic effects were good in the two groups (mean VAS < 3.0). The patients of group M had lower pain scores continually compared with group N and pain scores were statistically significant at 6, 12 and 24 hours. However, side effects occurred more frequently in group M. CONCLUSIONS: These results suggest that an adequate dosage of epidural morphine provides good analgesic effects and reduces the occurrence of side effects.