The significance of periventricular lucencies in the white matter on CT in demented patients is not understood. We studied the relationship of these changes to mental status of subjects with senile dementia of the Alzheimer type. A semiquantitative method showed more numerous and extensive lucencies in demented than in healthy elderly. Neuropathologic examination of five subjects with these changes and confirmed Alzheimer's disease revealed diffuse white matter pallor without infarction. There were no hypertensive vascular changes, although limited hyaline thickening was present.