Thirty cases of Alzheimer's disease and 30 age-matched controls were studied to determine the incidence of cerebral amyloid angiopathy and its relationship to age, neuritic plaque formation, and amyloid plaque content. Cerebral amyloid angiopathy (CAA) was present in 86% of AD cases and 40% of age-matched controls. Its frequent occurrence in AD is not merely a reflection of the advancing age of this group: it was seen only in the presence of neuritic plaques, regardless of age, and represents an integral component of AD. Neuritic plaques however, did occur in the absence of CAA in 17% of all cases. The amount of vascular and plaque amyloid tended to be of comparable severity in many cases, but significant discrepancies were observed, with preferential deposition of amyloid in either plaque or vessel. Our results suggest that neuritic plaque formation and amyloid deposition are linked genetically or etiologically, but independently expressed, without a cause-and-effect relationship.