BACKGROUND Spectral entropy of the electroencephalogram (EEG) has been proposed to monitor anaesthetic depth. We investigated the effect of nitrous oxide on response (RE) and state entropy (SE) of the EEG during lumbar disc surgery under anaesthesia with sufentanil and sevoflurane. METHODS In an open study, anaesthesia was induced with propofol and sufentanil, and maintained with 2% end-tidal sevoflurane concentration in air/oxygen (FiO2 = 0.4) in 25 patients. During surgery, nitrous oxide was randomly administered either at 0 or at 60% end-tidal concentration in 10 (control group) and 15 patients (nitrous oxide group), respectively. RE and SE were recorded at 2.5 min intervals for 10 min before randomization and for 25 min either continuously (control) or after achieving the target nitrous oxide concentration. RESULTS Two patients who received nitrous oxide were excluded from statistical analysis because of protocol violation. Nitrous oxide provoked a significant decrease in RE and SE from 46.2 +/- 11.1 and 44.3 +/- 11.1 to a lowest value of 27.8 +/- 8.3 and 27.1 +/- 8.9, respectively. The decrease in entropy persisted during the 25 min recording period. CONCLUSIONS Addition of nitrous oxide during balanced anaesthesia with sufentanil and sevoflurane provokes a decrease in response and state entropy of the EEG during lumbar disc surgery.