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The cerebral state index (CSI) is a low-cost and compact electroencephalographic monitor alternative to the bispectral index (BIS). Although both indices are shown as the number between 0 and 100, owing to the different calculation algorithms, both indices might be different during anaesthesia. The present study was performed to compare the BIS and CSI during sevoflurane–nitrous oxide anaesthesia.Forty patients scheduled for abdominal surgery aged 30–70 years were enrolled. The electrodes of both the cerebral state monitor and BIS were attached simultaneously. Anaesthesia was induced with midazolam and fentanyl, and was maintained with sevoflurane with 50% nitrous oxide in oxygen and intermittent epidural mepivacaine. During surgery, end-tidal sevoflurane concentration was kept at 0.5, 1.0, and 1.5% for more than 10 min to measure CSI and BIS.The duration of signal disturbance by electric cautery was 12.6 ± 4.2 min in the BIS and 3.9 ± 1.8 min in the cerebral state monitor (P < 0.05) for 9.5 ± 4.3 min duration of electric cautery. The recovery time from disturbance by electric cautery was 37 ± 12 s in the BIS and 4 ± 2 s in the cerebral state monitor (P < 0.05). The absolute values of the BIS and CSI were not significantly different. Both BIS and CSI decreased significantly when end-tidal sevoflurane concentration increased from 0.5 to 1.0%, but did not significantly change when end-tidal sevoflurane concentration increased from 1.0 to 1.5%. The BIS and CSI correlated well. The Bland–Altman plot showed good agreement between the BIS and CSI.During sevoflurane–nitrous oxide anaesthesia, the CSI was not significantly different from the BIS in the awake state and with sevoflurane concentration 0.5–1.5%.