The association between diabetes and the risk of developing dementia has received much attention in epidemiological studies. An accurate population-based prospective cohort study has been conducted in the elderly population of the town of Hisayama in Japan since 1985 aiming to elucidate the secular trends in the prevalence of dementia and examine risk and protective factors for dementia in the Japanese population. The prevalence of all-cause dementia significantly increased from 1985 to 2012. In regard to subtypes of dementia, a similar trend was observed for Alzheimer's disease (AD). In a prospective study of risk factors for dementia in Hisayama elder residents without dementia, diabetes was identified as a significant risk factor for developing all-cause dementia, especially AD. Moreover, 2-hour post-load glucose levels were closely associated with increased risk of all-cause dementia, AD, and vascular dementia. In a pathological study of Hisayama residents, higher levels of 2-hour post-load glucose, fasting insulin, and homeostasis model assessment of insulin resistance (HOMA-IR) were significantly associated with increased risk of neuritic plaques. The steep increase in the frequency of diabetes could lead to the increasing trend in the prevalence of dementia, especially AD, in the Japanese elderly.