Structural analysis of the capillary plexus in the brains of 5 patients with clinical and neuropathological diagnosis of Senile Dementia, Alzheimer Type, revealed a group of striking physical alterations compared with tissue specimens from 5 age-matched controls. Capillary walls were thickened with irregular lumpy, nodulated contours which appeared due, in part, to infiltration of the vascular wall with rounded cell-like bodies. In some cases, these resembled the perikarya of pericytes or monocytes, or their protoplasmic extensions, which were often filled with lipids. In each case, there was no trace of the perivascular neural plexus which regularly invests the microvasculature of the brain parenchyma. The loss of this neural plexus, believed to originate largely from locus ceruleus and the basal forebrain, may be related to the changes in capillary wall structure, and these, in turn, may lead to profound alterations in blood-brain barrier function. We suggest that this subcortically induceddenervation microangiopathymay serve as a pathogenic factor in the development of SDAT.