Abstract： Background In recent years, increasing numbers of patients are accepting neoadjuvant chemotherapy before their operation in order to get a better prognosis. But chemotherapy has many side-effects. We have observed that patients who accepted neoadjuvant chemotherapy are more sensitive to anesthetics. The aim of this study was to determine the median effective dose (EC50) of intravenous anesthetics for neoadjuvant chemotherapy patients to lose consciousness during target-controlled infusion.Methods Two hundred and forty breast cancer patients undergoing elective operations were assigned to six groups according to treatment received before their operation and the use of intravenous anesthetics during anesthesia;non-adjuvant chemotherapy+propofol group (group NP, n=40), Taxol+propofol group (group TP, n=40),adriamycine+cyclophosphamide+5-Fu+propofol group (group CP, n=40), non-adjuvant chemotherapy+etomidate group (group NE, n=40), taxol+etomidate group (group TE, n=40), adriamycine+cyclophosphamide+5-Fu+etomidate group (group CE, n=40). We set the beginning effect-site concentration (Ce) of propofol as 3.0 μg/ml and etomidate as 0.2μg/ml. The concentration was increased by steps until the patient was asleep, (OAAS class Ⅰ-Ⅱ), then gave fentanyl 3μg/kg and rocuronium 0.6 mg/kg and intubated three minutes later. The patients' age, height, and weight were recorded.BIS was recorded before induction, at the initial effect-site concentration and at loss of consciousness. The effect-site concentration was recorded when patient lost consciousness.Results There were no significant differences between groups in general conditions before treatment; such as BIS of consciousness, age, sex and body mass index. The EC50 of propofol in the NP, TP and CP groups was 4.11 μg/ml (95%CI: 3.96-4.26), 2.94 μg/ml (95% CI: 3.36-3.47) and 2.91 μg/ml (95% CI: 3.35-3.86), respectively. The EC50 of etomidate in the NE, TE and CE groups was 0.61 μg/ml (95% CI: 0.55-0.67), 0.38 μg/ml (95% CI: 0.33-0.44), and 0.35 μg/ml (95%CI: 0.34-0.36), respectively. There was no significant difference of BIS level before induction or in BIS50 level in any group when patients lost consciousness.Conclusions The EC50 of intravenous anesthetics to cause loss of consciousness in neoadjuvant chemotherapy groups is lower than in the control group. There was no significant difference of BIS level at which patients lost consciousness.