Caesarean section carries a high risk of awareness, especially in the period prior to neonatal delivery. We investigated the concentration of sevoflurane required to maintain bispectral index (BIS) < 60 until delivery occurred. We enrolled 23 parturients into an up-down sequential allocation study. The median effective end-tidal concentration (EC(50)) of sevoflurane was defined as that which maintained BIS < 60 between skin incision and delivery in 50% of patients. This was calculated using Dixon and Massey's method. Receiver operating characteristic curve analysis was used to establish BIS response probability thresholds. The EC(50) for sevoflurane was 1.22% (1.08-1.33, 95% CI). The probability of maintaining BIS < 60 was < 55% at a concentration of < 1.1%; this increased to 80% at concentrations of 1.2-1.3%. We conclude that sevoflurane concentrations of at least 1.2-1.3% should be administered in uncomplicated Caesarean section, so as to minimise the risk of awareness and recall.