Estradiol is a steroid-structured hormone that has a periodic rhythm in the menstrual cycle. We aimed to evaluate the interference of high estradiol levels and the depth of anaesthesia. The study was performed on 44 females undergoing gynaelocologic surgery. Blood samples were performed for estradiol level before the procedures. BIS scores were recorded at 5-min intervals after induction and during the operation. Cases were assigned to three groups: Group 1 (n:17) estradiol levels at or under 100 mu g/dL, Group 2 (n:14) levels were between 100 and 200 mug/dL and Group 3 (n:13) levels were above 200 mug/dL. Estradiol levels were found to be 59.94 +/- 23.59 mu g/dL in Group 1, 138.60 +/- 23.49 mug/dL in Group 2 and 239.30 +/- 41.08 mug/dL in Group 3. Significant differences were found between initial control and 10 and 80 min BIS levels. Anaesthetic consumption showed a decreased tendency in high estradiol cases. We concluded that an advanced clinical series should be performed to fully define the relationship between estradiol levels and anaesthesia depth.