To elucidate risk factors for cerebral amyloid angiopathy (CAA) in the elderly, we have investigated 201 autopsy cases of elderly Japanese (ages: 62-104 years), including 82 patients with Alzheimer's disease (AD). Severity of CAA showed no relationship with the history of hypertension, hyperlipidemia, or diabetes mellitus, nor with severity of atherosclerosis of cerebral and systemic arteries, indicating that common vascular risk factors would not be related to CAA. Incidence and severity of CAA were significantly higher in the AD cases compared with the non-AD cases (p < 0.0001). Severity of CAA correlated with densities of senile plaques and neurofibrillary tangles in total and non-AD cases, although the correlations were not significant within the AD cases. Associations of genetic polymorphisms with CAA have been investigated for genes of apolipoprotein E (APOE), presenilin 1 (PS1), alpha1-antichymotrypsin (ACT), butyrylcholinesterase, alpha2-macroglobulin, and paraoxonase. Severity of CAA in APOE epsilon4 carriers is significantly higher than that in non-epsilon4 carriers in total cases, although no significant difference was found in the CAA severity between the epsilon4 carriers and non-epsilon4 carriers within the AD or non-AD group. An intronic polymorphism of PS1 was significantly associated with the severity of CAA, indicating that the PS1 2/2 genotype may be related to lower risk of CAA. A polymorphism in the signal peptide sequence of ACT was significantly associated with the CAA severity in the AD group. Our results suggest that CAA shares risk factors with AD and that multiple genetic factors would be associated with the risk of CAA in the elderly.