We carried out two separate epidemiological studies on long-term changes, 10 years apart, on the prevalence rate of dementia in the elderly by the same method for the same area in Japan. We also performed a genetic study of patients with dementia of the Alzheimer type (DAT) based on the epidemiological studies. The number of patients with dementia was much larger in 1990 than in 1980. Especially, the number of mildly demented patients was significantly larger in 1990 than in 1980. The 35 patients with DAT did not show any mutations of amyloid-beta protein precursor, presenilin 1 and presenilin 2 genes. The frequency of apolipoprotein E (apo E) sigma4 allele in DAT was significantly higher than that in control subjects (p < 0.005). This study suggests that the frequency of DAT may increase by aging of the population in the future and we confirm the close association between apoE sigma4 allele and DAT in a community-based study in Japan.