BACKGROUND: Video assisted thoracoscopic surgery (VATS) can be performed under epidural anesthesia. However, epidural anesthesia dose not provide complete analgesia during VATS. In that case, remifentanil may provide adjuvant analgesia. The aim of this study was to find safe and effective target concentration of remifentanil during VATS under epidural anesthesia. METHODS: Patients were randomly allocated to target concentration of remifentanil 1 or 2 microgram/ml (Group N1 or N2). Thoracic epidural catheter was inserted at T_6-7 or T_7-8 and 0.75% ropivacaine 6 ml was administered. Patients were positioned in lateral position and remifentanil infusion was started. If patients complained of pain, target concentration of remifentanil was elevated by 0.5 microgram/ml. Hemodynamic and respiratory variables were checked during anesthesia. The degree of sedation was evaluated with Ramsey sedation scale and modified Steward recovery scale was used to evaluate recovery. RESULTS: There was no significant difference between 2 groups in hemodynamic and respiratory variables. N2 group showed more sedation on skin incision (P < 0.05). The incidence of pain complaint in N1 group was higher than those of N2 group (P < 0.05). There was no significant difference in the incidence of side effects and ephedrine administration. CONCLUSIONS: Remifentanil target concentration of 2 ng/ml is more effective than 1 ng/ml to reduce pain without significant side effects during VATS under thoracic epidural anesthesia.