BACKGROUND: Recent studies suggest that additional use of alfentanil could provide the best condition for the laryngeal mask airway (LMA) insertion. The aim of this study is to compare the median effective dose (ED50) of propofol for the classic LMA insertion and the insertion condition following between fentanyl and alfentanil adjuvant. METHODS: We enrolled 53 patients scheduled for minor surgery under general anesthesia. Patients were randomly allocated to the fentanyl group (n = 24) and the alfentanil group (n = 29) in double blind manner. For fentanyl group, 1microgram/kg of fentanyl was injected intravenously 90 sec before propofol. The afentanil group received 4microgram/kg of alfentanil and propofol coincidently. The insertion of LMA was attempted 90 sec after propofol administration. In accordance with Dixon's up-and-down method, the dose of propofol for consecutive patients in each group was varied with increments or decrements of 0.5 mg/kg based on the previous insertion results of patients. RESULTS: In the fentanyl and alfentanil group, the ED50 of propofol for LMA insertion according to Dixon's method was 2.0 +/- 0.3 mg/kg and 1.8 +/- 0.3 mg/kg, respectively. In addition, the ED50 of propofol of the fentanyl and alfentanil group according to probit regression model, 1.7 mg/kg (95% confidence interval, 1.2-2.2) and 1.7 mg/kg (1.3-2.0) were calculated respectively. There is no significant difference between the two groups. CONCLUSIONS: There was no significant difference in propofol ED50 for insertion of LMA and insertion condition between the alfentanil and the fentanyl group.