Objective To determine the half-effective target effect-site concentration (EC50 ) of sufentanil inhibiting the urethroscope insertion response when combined with propofol by target-controlled infusion (TCI) in the elderly patients undergoing transurethral resection of prostate (TURP) . Methods Twenty-two ASA Ⅰ- Ⅲ patients, aged 65 -79 yr, weighing 47-81 kg, undergoing TURP, were enrolled in the study. Anesthesia was performed with TCI of propofol and sufentanil. The target plasma concentration of propofol was 4 μg/ml. The target effect-site concentration of sufentanil was determined by up-and-down sequential trial. The initial target effect-site concentration of sufentanil was 0.3 ng/ml and the ratio of the target concentrations between the two consecutive patients was 1.1. The ECW and 95% confidence interval of sufentanil required to inhibit the response to urethroscope insertion were calculated. Results The EC50 of sufentanil required to inhibit the urethroscope insertion response was 0.23 ng/ml (95% confidence interval 0.12-0.44 ng/ml). Conclusion When combined with propofol by TCI (target plasma concentration 4 μg/ml), the EC50 of sufentanil inhibiting the response to urethroscope insertion is 0,23 ng/ml in the elderly patients undergoing TURP.