Primary cerebrovascular amyloidosis resulting in significant cerebral parenchymal damage was encountered in 23 autopsied cases at the Mayo Clinic over the past 10 years. Patients were 60 to 97 years old and both sexes were equally represented. Large- and medium-sized leptomeningeal and cortical arteries showed the characteristic pattern of medial and intimal involvement, with luminal stenosis. The walls of smaller arteries were often diffusely infiltrated, with fibrinoid degeneration and miliary aneurysm formation. The amyloid nature of the infiltrate was confirmed by electron microscopic examination in all cases. All cases showed varying numbers of perivascular or independent senile plaques in the cerebral cortex. Alzheimer's neurofibrillary tangles were absent or were limited to the hippocampal region in all but two cases. Multiple, small cortical infarcts and hemorrhages were regularly present. Larger hemorrhage was present in nine cases. Of nine patients with terminal massive cerebral hemorrhage, only two were hypertensive. Six patients had had progressive dementia; four had had single episodes of vascular events and seven, multiple episodes; and four had had both dementia and vascular episodes. Primary cerebral amyloid angiopathy should be regarded as an important cause of mental deterioration and fatal cerebrovascular accidents in the elderly.