We performed a quantitative neuropathological investigation of senile plaques and neurofibrillary tangles in the brains of 14 Japanese patients with diffuse Lewy body disease (DLBD), and examined apolipoprotein E (APOE) gene polymorphism in these patients. Most DLBD brains had as many senile plaques as those with Alzheimer-type dementia (ATD), but fewer neurofibrillary tangles. The APOE epsilon 4 allele frequency in DLBD was 39.3%, similar to that previously reported in pathologically diagnosed ATD. DLBD and ATD are clinically and pathologically distinct, but may have common mechanisms with regard to the formation of amyloid deposition based on these findings. Future investigation of the pure form of DLBD may clarify the association between Lewy body dementia and APOE.