The author investigated the relations of brain atrophy demonstrated by computerized tomography, dementia and electroencephalogram (EEG) in presenile and senile diseases. The result revealed that the correlation of brain atrophy, when demonstrated on the basis of ventricular ratio, is more definite and higher with dementia than that when demonstrated on the ventricular width. It was also revealed that in the control group, the ventricle gets enlarged due to aging, but no such tendency was observed in the patient group. In the case of septugenarians, no significant difference was observed between the two, and the ventricular-brain ratio or the criteria of brain atrophy is 8 -- 9% in patients in their forties to fifties; 12% for those in their sixties, and more than 15% among those in their seventies. In the relation of the brain atrophy to EEG, it was revealed that EEG is closely related to dementia than is the ventricular ratio. Upon following the progress of patients with Alzheimer, it was found that dementia and brain atrophy do not progress parallel to each other, but in stages.