Background: Bispectral index (BIS) is considered very useful to guide anesthesia care in elderly patients, but its use is controversial for the evaluation of the adequacy of analgesia.This study compared the BIS changes in response to loss of consciousness (LOC) and loss of somatic response (LOS) to nociceptive stimuli between elderly and young patients receiving intravenous target-controlled infusion (TCI) of propofol and remifentanil.Methods: This study was performed on 52 elderly patients (aged 65-78 years) and 52 young patients (aged 25-58 years), American Society of Anesthesiologists physical status Ⅰ or Ⅱ.Anesthesia was induced with propofol administered by TCI.A standardized noxious electrical stimulus (transcutaneous electrical nerve stimulation, [TENS]) was applied (50 Hz, 80 mA, 0.25 ms pulses for 4 s) to the ulnar nerve at increasing remifentanil predicted effective-site concentration (Ce) until patients lost somatic response to TENS.Changes in awake, prestimulus, poststimulus BIS, heart rate, mean arterial pressure, pulse oxygen saturation, predicted plasma concentration, Ce of propofol, and remifentanil at both LOC and LOS clinical points were investigated.Results: BISLOC in elderly group was higher than that in young patient group (65.4 ± 9.7 vs.57.6 ± 12.3) (t =21.58, P ＜ 0.0001) after TCI propofol, and the propofol Ce at LOC was 1.6 ± 0.3 μg/ml in elderly patients, which was significantly lower than that in young patients (2.3 ± 0.5 μg/ml) (t =7.474, P ＜ 0.0001).As nociceptive stimulation induced BIS to increase, the mean of BIS maximum values after TENS was significantly higher than that before TENS in both age groups (t =8.902 and t =8.019, P ＜ 0.0001).With increasing Ce of remifentanil until patients lost somatic response to TENS, BISLOS was the same as the BISLOC in elderly patients (65.6 ± 10.7 vs.65.4 ± 9.7), and there were no marked differences between elderly and young patient groups in BISawake, BISLOS, and Ce of remifentanil required for LOS.Conclusion: In elderly patients, BIS can be used as an indicator for hypnotic-analgesic balance and be helpful to guide the optimal administration of propofol and remifentanil individually.