We describe a Japanese family with parkinsonism and later-onset dementia. The proband developed parkinsonism at the age of 61 years, followed by dementia starting when she was 67. Her uncle, who was also her husband, died at the age of 78 years after 7- and 5-year histories of parkinsonism and dementia, respectively. Pathological examination of these two patients showed marked neuronal loss with Lewy bodies (LBs) in the brain stem pigmented nuclei and numerous cortical LBs and ubiquitin-positive hippocampal CA2/3 neurites were observed. The proband also had many amyloid plaques. Their two sons developed similar parkinsonism at the ages of 39 and 28 years and also suffered later-on-set dementia. The apolipoprotein E genotype of the proband, her uncle and one of their sons was epsilon3/4 and that of the other son was epsilon4/4. These findings strongly suggest that this family has autosomal dominant diffuse LB disease.