Sixty-six patients with tumours in the sellar region were examined. All were operated on either by the transfrontal or the transsphenoidal route. Pre- and postoperative longitudinal electroencephalographic investigations were performed. Preoperative electroencephalograms showed a normal frequency content in cases of intrasellar tumours or those reaching the chiasma. Nearly all cases had irregularities in the temporal regions. Tumours compressing the third ventricle had slower average frequencies and a general slowing in all cases. Besides these alterations unilateral delta waves or bitemporal dysrhythmic groups were sometimes found. A connection between extension of the tumour and its histological nature could not be found, but the operative approach influenced the electroencephalographic disturbances enormously. After a transfrontal approach and removal of the tumour the electroencephalogram was unchanged. Sometimes a mild transient bitemporal slowing was present. But after a transfrontal operative approach a general slowing was common, usually with focal marked slow activity in the right fronto-temporal area.