Background: Airway management of the obese patient presenting for surgery is more likely to be a challenging problem. Supraglottic airway device has been adopted as a bridge to connect ventilation and tracheal intubation in obese patients who would be suffered with difficult intubation. The optimum sevoflurane concentration for supraglottic airway device insertion allowing spontaneous breathing in 50% of obese patients (ED50) is not known. The purpose of this study was to determine the ED50 of sevoflurane for supraglottic airway device Blockbuster (TM) insertion with spontaneous breathing in obese patients requiring general anesthesia. Methods: Thirty elective obese patients (body mass index 30-50 kg/m(2)) undergoing bariatric surgery were recruited in this study. The predetermined target sevoflurane concentration (initiating at 2.5% with 0.5% as a step size) was sustained for >5 min using a modified Dixon's up-and-down method, and then the supraglottic airway device lockbuster (TM) was inserted. The patient's response to supraglottic airway device insertion was classified as either 'movement' or 'no-movement'. The ED50 of sevoflurane were determined by calculating the midpoint concentration of crossover point from 'movement' or 'no-movement' response. Results: The ED50 of sevoflurane for supraglottic airway device Blockbuster (TM) insertion in obese patients calculated using up-and-down method were 2.50 +/- 0.60%. The ED50 and ED95 (95% confidence interval) obtained by probit regression analysis were 2.35 (1.28-3.42) % and 4.03 (3.16-17.83) % for supraglottic airway device Blockbuster (TM) insertion, respectively. Conclusion: We conclude that the optimum end-tidal sevoflurane concentration required for the supraglottic airway device Blockbuster (TM) insertion allowing spontaneous breathing in 50% of obese patients (ED50) is 2.5 +/- 0.6%.