BACKGROUND: Butorphanol, a synthetic partial agonist-antagonist narcotic, produces adequate analgesia for postoperative pain. This study was designed to determine the appropriate dosage of butorphanol when administered with ketorolac by intravenous patient controlled analgesia (IV-PCA) after subtotal gastrectomy. METHODS: Ninety ASA physical status I or II patients undergoing subtotal gastrectomy were randomly allocated into one of three groups according to type of drug used (n = 30 for each group). The patients were divided into group B6 (butorphanol 6 mg), group B10 (butorphanol 10 mg) and group B14 (butorphanol 14 mg). Drugs for each group were mixed with 300 mg of ketorolac and normal saline (total amount: 100 ml) for infusion. Bolus dose, maintenance dose and lockout interval were 0.5 ml per each press, 1 ml/hr and 15 minutes, respectively. In each group, numerical rating scale (NRS) score, sedation score and incidence of side effect were checked. RESULTS: There were no significant differences in analgesic effects and sedation score among three groups but the NRS score of group B14 is lower than that of group B6 (P < 0.05) 3 hrs after the recovery room. CONCLUSIONS: We recommend 6 mg butorphanol, mixed with 300 mg of ketorolac, and normal saline for postoperative pain relief using IV-PCA.