BACKGROUND: Inhalation anesthetics have been known to be effective in lowering respiratory system resistance (Rrs); however the bronchodilation effects of sevoflurane has not been well defined. We administered same end-tidal concentration of halothane, isoflurane and sevoflurane and continuous thiopental infusion to compare bronchodilation effects between these groups. METHODS: After institutional reviewed board approve and written informed consent, 59 patients (smoker 47, non-smoker 12) were randomized to receive either 1.1 end-tidal concentration of inhalation anesthetics (Halothane, Isoflurane, Sevoflurane) in 100% O2 or continuous thiopental infusion (15 mg/kg/hr) in 50% N2O/50% O2. Patients were anesthetized with 2 microgram/kg fentanyl and 5 mg/kg thiopental. Tracheal intubation was done with 1 mg/kg succinylcholine and paralyzed patient with vecuronium (0.1 mg/kg IV). Just after intubation baseline Rrs, blood pressure, heart rate were measured with Datex Capnomac (Capnomac Ultima, Datex, Finland). Just after baseline measurement we administered anesthetics and measured same variables repeatedly at 5 min, 10 min following given anesthetics. Data were analyzed using analysis of variance and t-test to determine bronchodilation effects of Rrs between groups. RESULTS: Rrs at 5 min, halothane and sevoflurane decreased more significantly than isoflurane (33.4%, 38.5%, 23.2%) but increased in thiopental infusion (10.6%). Rrs at 10 min, halothane and sevoflurane decreased (39.1%, 41.6%) and less decreased in isoflurane (26.5%), but more increased in thiopental infusion (13.1%) (Table 2, Fig. 2). CONCLUSIONS: In lowering respiratory system resistance, sevoflurane decreases resistance as much as halothane and is more potent than isoflurane and thiopental which was induced by tracheal intubation.