(1) In a series of 67 cases with unequivocal pneumoencephalographic evidence of either cortical atrophy or ventricular dilatation, many normal EEGs were found as well as a considerable variety of abnormal electroencephalographic patterns.(2) Those cases of cortical atrophy or ventricular dilatation having convulsive seizures as part of the clinical picture invariably showed abnormal EEGs. In the majority of such cases the electroencephalographic abnormalities were of a marked degree.(3) In cases free of convulsive seizures, the electroencephalographic abnormalities were generally of a mild degree, and the incidence of abnormality was similar for cases with ventricular dilatation (44%) and cortical atrophy (37%). However, more slow activity was found in cases with ventricular dilatation than in cases with cortical atrophy, and more fast activity in the latter group than in the former.(4) Asymmetrical electroencephalographic abnormalities were recorded in only 5 individuals, 2 of whom had unequal ventricular enlargement. In these 2, the greater electroencephalographic abnormality was on the side of the smaller ventricle.Abstract Teaser