Patients with multi-infarct dementia often have periventricular low density lesions on computed tomography and periventricular hyper-intensity lesions on computed tomography and periventricular hyper-intensity lesions on magnetic resonance imaging. Hachinski called this condition leukoaraiosis and results of previous studies that in multi-infarct dementia leukoaraiosis correlates with cerebral hypoperfusion. Periventricular low-density lesions in the white matter also occur, but their origin and clinical significance insuch patients is unknown. We studied when these lesions develop in patients with Alzheimer's dementia, and whether their presence correlates with clinical findings and cerebral blood flow. The subjects were 37 patients with a probable diagnosis of Alzheimer's dementia, as based on the Neuroepidermilogy Branch of the National Institute of Neurological Disorders and Stroke and the Alzheimer's disease and Related Disorders Association system. Autopsy findings were also available for 2 patients. Patients at higher Functional Assessment Staging of Senile Dementia of Alzheimer Type stages had more extensive periventricular low-density lesions. Patients with the lesions were more likely to have grasp reflex and sucking relex. Blood flow to the frontal and parietal cortices was significantly less in patients with the lesions than in those without the lesions. Neither of the 2 autopsies cases yielded evidence of arteriolosclerosis. Periventricular low-density lesions in Alzheimer's dementia may be closely associated with a degenerative process different from that seen in multi-infarct dementia.