The term Lewy body disease (LBD) was proposed earlier to describe a disease classified into three types (A, B, and C) according to the distributional pattern of Lewy bodies in the CNS. Group A (diffuse type of LBD) shows clinical symptoms of "dementia-parkinsonism syndrome". The most remarkable pathologic feature is the widespread appearance of numerous Lewy bodies not only in the brain stem and diencephalon (as in group C), but also in the cerebral cortex and basal ganglia, which is complicated by senile changes of various degrees. Group B is the transitional type between groups A and C. Group C (brain stem type of LBD) is identical with idiopathic Parkinson's disease. In this paper, 12 of our cases with diffuse type LBD were studied clinicopathologically and compared with eight similar cases in the literature. The neuropathologic substrate of progressive dementia in this disease is also discussed. LBD is a clinicopathologic entity; the diffuse type of LBD, a special form of this disease, presents mainly a presenile dementia.