Clinical signs are unreliable for guiding anaesthetic administration and it is suggested that using the bispectral index can improve anaesthetic delivery. In the current study, isoflurane administration was guided to a bispectral index range of 55–60. Intra-operative responsiveness, as assessed by the isolated forearm technique, was compared with whether the bispectral index predicted/identified a patient's appropriate hand movements in response to commands. Thirty-four women underwent major gynaecological surgery with isoflurane/air and atracurium. Eleven women responded on 32 occasions with appropriate hand movements to commands given during surgery, of which the bispectral index detected 17 (sensitivity 53%). The bispectral index suggested consciousness 660 times in the absence of any movement responses (specificity 69%). The positive predictive value of the bispectral index was 3%. The median (IQR [range]) bispectral index value associated with an intra-operative response was significantly lower than that associated with eye opening after surgery: 60 (50–68 [36–83]) vs 77 (75–84 [59–90]), respectively (p = 2.25 × 10−8). Conversely, end-tidal isoflurane concentration was significantly higher at intra-operative response than at eye opening: 0.3 (0.3–0.4 [0.2–0.9]) vs 0.2 (0.1–0.2 [0.1–0.3]), respectively (p = 7.36 × 10−8). For patients who responded more than once during surgery, the bispectral index value associated with a response was not constant. No patient had recall for surgery or the taped commands, and only one could remember dreaming (a good dream). Titrating isoflurane to target a bispectral index range of 55–60 may result in an unacceptable number of patients who are conscious during surgery (albeit without recall).