Fourteen patients with NPH were studied with special reference to associated EEG changes, one case being reported in detail. In 13 cases the EEG was abnormal, the abnormality most frequently observed being a of the rhythmic slow wave type, consisting of bursts of monorhythmic theta or delta waves seen in both hemispheres. This type of abnormality was present in 7 cases. In 4 of the 5 cases with serial EEG's the abnormality became more accentuated with the advance of the illness. In 4 of 5 cases in whom postoperative EEGs were obtained there was significant improvement of the EEG after the shunting procedure. There were stron correlations between the EEG abnormality described, demonstrable ventricular reflux on cisternography, and the effectiveness of the shunt operation. It was considered that serial EEGs are a useful diagnostic procedure in following up patients at risk of developing NPH.