Objective:To evaluate the effect of obesity on the dose-effect relationship of remimazolam when combined with alfentanil in painless gastroscopy.Methods:American Society of Anesthesiologists physical status Ⅰor Ⅱ patients of both sexes, scheduled for elective painless gastroscopy, aged 18-64 yr, were divided into 2 groups according to the body mass index (BMI): normal (BMI 19-24 kg/m 2) group and obese (BMI≥28 kg/m 2) group.Alfentanil 5 μg/kg combined with remimazolam was given intravenously in all the patients, and the dose of remimazolam was determined by the modified Dixon′s up-and-down method.The initial dose of remimazolam was 0.25 mg/kg, and each time the dose was increased or decreased by 0.05 mg/kg based on the sedative effect.The response was defined as positive when the responses that affected the operation of examination developed during insertion of the gastroscope and within the first 2 min of examination such as swallowing, bucking or body movement.This process was repeated until the seventh intersection occurred.The 50% effective dose (ED 50), 95% effective dose (ED 95), and 95% confidence interval ( CI) of remimazolam were calculated by probit method. Results:There were 26 patients in normal group and 18 patients in obese group.The ED 50 (95% CI) of remimazolam was 0.196 (0.087-0.274) mg/kg, and the ED 95 (95% CI) was 0.322 (0.256-1.397) mg/kg in normal group.The ED 50 (95% CI) of remimazolam was 0.125 (0.102-0.148) mg/kg, and the ED 95 (95% CI) was 0.161 (0.141-0.242) mg/kg in obese group.The ED 50 and ED 95 were significantly lower in obese group than in normal group ( P<0.001). Conclusions:Obesity increases the potency of remimazolam when combined with alfentanil 5 μg/kg in the patients undergoing painless gastroscopy.