OBJECTIVE: To determine bispectral index (BIS) values produced by equipotent concentrations of commonly used volatile anaesthetics. METHODS: Female patients undergoing thyroidectomy were randomly assigned to receive isoflurane, sevoflurane or desflurane anaesthesia. After induction, anaesthesia was maintained by the volatile agent at 1 minimum alveolar concentration and supplemented with remifentanil infusion. BIS values were recorded during 1?h surgical anaesthesia after a 15?min equilibrium phase. RESULTS: Time-averaged BIS value during the study period was significantly lower in the desflurane group (n?=?29) than the sevoflurane group (n?=?27) (37.0?±?4.9 vs 41.5?±?5.9). Duration of deep hypnosis (BIS?<?40) was significantly longer in the desflurane group than the sevoflurane group (40.2?±?20.7 vs 24.3?±?22.5?min). There were no significant differences in any parameter between the isoflurane group (n?=?27) and any other group. CONCLUSIONS: Desflurane produces a greater hypnotic effect than sevoflurane during equipotent anaesthesia. Management of volatile anaesthesia using predetermined minimum alveolar concentration targets can lead to an unnecessarily long duration of deep hypnosis.