The frequency of headache occurring in the postoperative period following three common anaesthetic techniques is described. The incidence following anaesthesia with halothane and controlled ventilation was found to be significantly higher than in a similar group in which halothane was not used. The two groups were otherwise comparable in all respects. A third technique employing halothane and spontaneous respiration showed a still higher incidence of headache when compared with the halothane and controlled ventilation method, but this difference was not statistically significant. In ten patients (20 per cent of those receiving halothane) headache was the main postoperative complaint. The possible relationships between the headaches after halothane and the effect of this drug on the cerebral vasculature and cerebrospinal fluid pressure are discussed.