The brain from 98 consecutive patients with the clinical diagnosis of dementia were examined at autopsy in a standardized fashion. Alzheimer's Disease was present in 79 of the cases, 76%, but represented the only diagnosis in 41%. Thus, almost 60% had another associated pathologic disorder. Cerebral amyloid angiopathy (CAA) represented the single largest subset, present in 25 cases. 40% were accompanied by either 1) small, microscopic infarcts or cortical scars, or 2) small collections of macrophages containing hemosiderin or small hemorrhages. CAA occurred with both atherosclerotic cortical infarcts and arteriolosclerotic subcortical pallor or lacunar infarcts. Alzheimer's Disease occurred with Diffuse Lewy Body (DLB) Disease in 13 cases. DLB Disease did not occur as a distinct entity, and thus may represent the second largest subset of Alzheimer's Disease. Both Alzheimer's Disease and DLB Disease accounted for dementia in Parkinson's Disease. Almost 25% of all cases had a disorder other than Alzheimer's Disease.