BACKGROUND: Intravenous patient-controlled analgesia (IV-PCA) is a safe and effective method for post-operative pain control. But opioids used in IV-PCA result in some side effects such as respiratory depression, nausea, vomiting, itching and urinary retention. Nalbuphine, an agonist-antagonist, has a considerable analgesic effect without serious complications. This study was designed to evaluate the effectiveness of post-operative pain relief in PCA-administered nalbuphine-ketorolac and morphine-fentanyl-ketorolac. METHODS: Patients scheduled for total abdominal hysterectomy were randomly assigned in a double-blind manner into one of two groups. Group 1 (n=24) and Group 2 (n=28) received nalbuphine-ketorolac or morphine-fentanyl-ketorolac, respectively. All patients received same background infusion rate (2 ml/hr), PCA dose (0.5 ml) and lockout interval (15 min) just after peritoneum closure. And post-operative pain scores were recorded with numerical rating scale (NRS) at 1, 2, 6, 12, 24 and 48 hr after operation. RESULTS: The pain control effect in group 1 was more effective than in group 2. The patients' satisfaction was more superior in group 1. And the number of using PCA button was more frequent in group 2. CONCLUSION: This study suggests that intravenous nalbuphine is an excellent alternative to morphine and fentanyl for postopertive pain control.