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Background: The purpose of this study was to determine the clinical effective dose of alfentanil required for successful tracheal intubation during inhalation induction using 5% sevoflurane without neuromuscular blockade in children sedated with ketamine. Methods: Twenty-one children, aged 3−10 years, undergoing surgeries under general anesthesia were enrolled into the study. All patients were premedicated with 0.004 mg/kg glycopyrrolate intramuscularly 30 min before anesthesia. To facilitate separation of the child from the parents, intravenous 1 mg/kg ketamine was given prior to transfer into the operating room. After inhalation induction with 5% sevoflurane and 100% oxygen, pre-determined dose of alfentanil was injected over 20 sec. The dose of alfentanil was determined by modified Dixon's up-and-down method (2μg/kg as a step size starting from 12μg/kg). The study ended when six independent pairs of patients, who manifested cross over from ‘failure’ to ‘success’ for tracheal intubation, were recruited. Results: In 50% of children, the effective bolus dose (ED50) (95%confidence intervals) of alfentanil for successful tracheal intubation was 7.2μg/kg (6.3−8.1μg/kg) during sevoflurane inhalation induction. From isotonic regression, 95% effective dose (ED95)(95% confidence intervals) of alfentanil was 9.9μg/kg (2.2−16.0μg/kg). Conclusions: During inhalation induction using 5% sevoflurane without neuromuscular blockade after ketamine sedation, the ED50and ED95 of alfentanil for successful tracheal intubation were 7.2μg/kg and 9.9μg/kg in children, respectively.