Using immunohistochemical staining methods with antibodies to amyloid beta protein and human cystatin C, we examined cerebrovascular amyloid protein in the brains from 46 cases with cerebral amyloid angiopathy (seven with Alzheimer's disease, one with Down's syndrome, 18 with intracranial hemorrhage, 10 with cerebral infarction, and 10 elderly patients without any neurologic disorder). All cerebrovascular amyloid deposits in these 46 cases were consistently immunoreactive to anti-beta protein antibody. However, in nine cases some vascular walls with strong beta protein immunoreactivity also reacted less intensely with the anti-cystatin C antiserum. Of these nine cases, seven showed relatively heavy cerebrovascular amyloid deposition, and all seven had suffered a fatal subcortical hemorrhage presumably caused by cerebral amyloid angiopathy. Previous limited studies have suggested that the amyloid protein seen in elderly individuals with cerebral amyloid angiopathy is composed of beta protein. However, subcortical hemorrhage rarely occurs in such individuals. Our study shows that aged patients with different brain disorders commonly suffer from beta protein-type cerebral amyloid angiopathy, and we also suggest that the severity of beta protein-type cerebrovascular amyloid deposition is a fundamental factor in cerebral amyloid angiopathy-induced brain hemorrhage in the elderly. The nature of the cystatin C-immunoreactive substance in some of these vascular lesions is uncertain, but it might conceivably play an additional important role in the pathogenesis of brain hemorrhage in these cases.