Background: Sevoflurane has been shown to be suitable for inhalation induction of anesthesia in adults. The addition of remifentanil during sevoflurane induction allows for rapid and smooth tracheal intubation without muscle relaxant. The high concentration of remifentanil can result in improved conditions for tracheal intubation but also caused a cardiovascular instability. We tried to estimate optimal effect site concentration of remifentanil for tracheal intubation without muscle relaxants during inhalation anesthetic induction with 8 vol% sevoflurane and 50% N2O. Methods: One hundred five ASA I and II patients were randomly allocated to receive an effect site concentration of remifentanil 2, 4, or 6 ng/ml. Trachea was intubated without muscle relaxant after 3 minutes of inhalation induction with 8 vol% sevoflurane and 50% N2O. The intubating conditions for tracheal intubation were assessed and blood pressure and heart rate were recorded at the baseline, before intubation, just after intubation, 1, and 3 minute after intubation. Results: Tracheal intubation was successful in all patients. Clinically acceptable intubation conditions were 60%, 94%, and 100% in 2 ng/ml, 4 ng/ml, and 6 ng/ml group respectively. However, the number of patient who requested rescue medication for bradycardia in 6ng/ml group was significantly higher than that in the other group (P＜0.05). Conclusions: We concluded that 4ng/ml effect site concentration of remifentanil is adequate concentration for tracheal intubation without muscle relaxants during inhalation induction using sevoflurane. (Korean J Anesthesiol 2009; 56: 639~45)