Purpose: This study aimed to assess the effect of intravenous dexmedetomidine (DEX) on sevoflurane EC50 for supraglottic airway device (SAD) insertion in spontaneously breathing morbidly obese patients. Patients and methods: Thirty-eight morbidly obese patients with a body mass index 40-57 kg/m(2) who were scheduled for bariatric surgery under general anesthesia requiring tracheal intubation were randomly allocated to two groups receiving the different treatments: group S, saline was given intravenously, and group D, a bolus dose of DEX 1 mu g/kg was administered intravenously over 10 mins, followed by intravenous DEX infusion at a rate of 0.5 mu g/kg/h. Five percent sevoflurane was initially inhaled for anesthesia induction and then end-tidal expiratory sevoflurane concentration (ETsev) was adjusted to a target value as to the modified Dixon's up-and-down method. Patients' response to SAD insertion was classified as "movement" or "no movement". The average of the midpoints of all crossover points was defined as calculated sevoflurane EC50 for successful SAD insertion. Furthermore, the probit regression analysis was used to determine sevoflurane end-tidal concentrations where 50% (EC50) and 95% (EC95) insertions of SAD were successful. After the observation was completed, flexible bronchoscope-guided intubation was performed through the SAD. Results: The calculated sevoflurane EC50 for successful SAD insertion was significantly lower in group D than in group S (1.75 +/- 0.32% vs 2.92 +/- 0.26%, p<0.001). By the probit regression analysis, EC50 and EC95 of sevoflurane for successful SAD insertion were 1.59% (95% CI, 1.22-1.90%) and 2.15% (95% CI, 1.86-3.84%) in group D, respectively, and 2.81% (95% CI, 2.35-3.29%) and 3.32% (3.02-6.74%) in group S. Conclusion: When sevoflurane inhalational induction is performed in spontaneous breathing morbidly obese patients, intravenous DEX can reduce sevoflurane EC50 for successful SAD insertion by about 40%.