BACKGROUND: Naloxone is an opioid antagonist and effective in reducing and reversing opioid-related side effects. In addition, low-dose naloxone may reverse or potentiate the analgesic effect of an opioid. The present study was designed to examine the analgesic efficacy and side effects of the combination of naloxone with morphine in patients using intravenous PCA (patient-controlled analgesia). METHODS: Patients were randomly assigned to receive one of three PCA regimens: group 1 (40 mg morphine + 90 mg ketorolac + 2.5 mg dorperidol), group 2 (40 mg morphine + 90 mg ketorolac + 0.8 mg naloxone), or group 3 (40 mg morphine + 90 mg ketorolac + 1.6 mg naloxone). All patients were given an initial loading dose of 0.1 mg/kg morphine at the end of surgery. Pain scores, side effects, and overall satisfaction were assessed at 30 min, 1, 8, 24 and 48 hr postoperatively. Blood pressure, heart rate, and respiratory rate were also monitored for 48 hours. RESULTS: The pain score was significantly lower in group 2 than in group 1 and group 3 at 8 hr and 24 hr postoperatively. Cumulative morphine usage during 48 hr was the least in group 2 compared with group 3. There were no differences in the overall incidence of side effects, patient satisfaction, and hemodynamic parameters among the groups. CONCLUSIONS: The present results suggest that a low-dose naloxone with intravenous morphine PCA is effective in reducing opioid-related side effects and in increasing the quality of analgesia.