The association between diabetes mellitus and the risk of developing dementia has received much attention in epidemiological studies. However, the results from these studies are still inconclusive. An accurate population‒based prospective cohort study of dementia was begun in 1985 in the town of Hisayama, Japan. To examine the association between the glucose tolerance status and development of dementia, a total of 1,017 community‒dwelling dementia‒free subjects aged≥60 years who underwent a 75‒g oral glucose tolerance test were followed up for 15 years. We demonstrated that people with diabetes were at an increasing risk of Alzheimer’s disease（AD）and vascular dementia（VaD）. Moreover, 2‒hour post‒load glucose levels were closely associated with an increased risk of AD and VaD, but no such associations were observed for fasting plasma glucose levels.