BACKGROUND: Thoracotomy is considered one of the most painful surgical procedures. Thus optimal pain control is essential in the postoperative care of thoracotomy patients. In this randomized double-blinded study, we sought to determine an optimal dose-combination of sufentanil and ropivacaine 0.2% for postoperative epidural analgesia. METHODS: Forty-five of fifty-two patients that underwent thoracotomy were assigned to three groups to receive continuous high thoracic epidural analgesia with ropivacaine 0.2% + sufentanil 0.5microgram/ml (group I), ropivacaine 0.2% + sufentanil 0.75microgram/ml (group II), or ropivacaine 0.2% + sufentanil 1.0microgram/ml (group III). Upon completion of surgery, a thoracic epidural catheter was inserted at the T1-3 level, and moved downward by 5-7 cm. Epidural continuous infusion was administered at 5 ml/hr. Resting VAS (visual analogue scale), dynamic VAS, forced vital capacity, and side effects were recorded at three times: immediate after extubation; 12 hours after arrival at the ICU; and 24 hours after arrival at the ICU. RESULTS: Resting VAS was significantly lower in group III immediately after extubation (P < 0.05) and dynamic VAS was significantly lower in group III immediately after extubation, and 12 hours after arriving at the ICU (P < 0.05) than in the other two groups. The differences in ketorolac dosing number, sedation scores, respiratory depression, pruritus, nausea/vomiting, and motor block were not significant between the three groups. CONCLUSIONS: We conclude that the combination of ropivacaine 0.2% and sufentanil 1.0microgram/ml provides effective analgesia for post-thoracotomy pain with fewer side effects than the other two combinations tested.