BACKGROUND: Intubation of the trachea frequently results in a rise in respiratory system resistance (Rrs) that is reversed by inhaled bronchodilators. In asthmatics, this reflex may occasionally result in profound bronchoconstriction, and anesthesiologists often try to avoid tracheal intubation if possible in asthmatics. The hypothesis of this study was that insertion of a laryngeal mask airway (LMA) would be less likely to result in reversible bronchoconstriction than would insertion of an endotracheal tube (ETT). METHODS: A total of 52 (45 male, 7 female) patients were randomized to placement of a 7.5 mm (females) or 8.0 mm (males) endotracheal tube or a #4 (females) or #5 LMA (males). Anesthesia was induced with 2 microgram/kg fentanyl and 5 mg/kg thiopental and airway placement facilitated with 1 mg/kg succinylcholine. After ensuring that a seal to greater than 20 cmH2O existed, Rrs was measured immediately following airway placement using the isovolumic method during positive pressure ventilation with oxygen. Correction was made for the resistance of the ETT but not for the resistance of the LMA, which was insignificant at the flows used. Inhalation anesthesia was then begun with isoflurane (ISF) to achieve an end-tidal concentration of 1.0% for ten minutes. Rrs was then measured again under identical conditions. In the LMA patients, fiberoptic laryngoscopy was then performed to ensure that the scope could be passed to the level of the cords without epiglottic obstruction. RESULTS: Among LMA patients, the initial Rrs was significantly lower than among ETT patients (9.2 0.7 vs 13.4 1.9 cmH2O/L/s, P <0.05). After 10 minutes of ISF, the resistance declined to 8.6 0.7 in the ETT group but remained unchanged at 9.1 0.7 cmH2O/L/s in the LMA group. The decline in Rrs in the ETT group of 4.7 1.4 cmH2O/L/s was highly significant compared to the lack of change in the LMA group (P <0.01). CONCLUSIONS: Despite the inclusion of the resistance of the LMA and the laryngeal resistance, Rrs in LMA patients was still clearly lower than in ETT patients. Furthermore, resistance dropped rapidly only in ETT patients after ISF, a potent bronchodilator, suggesting that reversible bronchoconstriction was present in ETT patients but not LMA patients. We conclude that an LMA is a better choice of airway to minimize airway reaction.