In this article, we have reviewed the findings of prospective cohort studies throughout the world to examine influences of lifestyle-related diseases on the risks of total dementia, Alzheimer disease (AD), and vascular dementia (VD). In some cohort studies on elderly populations, diabetes was found to be consistently associated with the risk of VD, but was inconsistently associated with the risk of AD. A cohort study on the elderly residents of the town of Hisayama, Japan, revealed the significant associations between glucose intolerance and the risks of both VD and AD. Clinical and experimental evidence has indicated that glucose intolerance and diabetes induce dementia through various mechanisms such as atherosclerosis,microvascular disease, glucose toxicity, and impaired insulin metabolism. No cohort studies have indicated significant associations between late-life hypertension and the risk of AD, and only 1 study has revealed the significant influence of hypertension on the risk of VD in an elderly population. The Hisayama study revealed that late-life hypertension was a significant risk factor for the development of VD but not for AD. A few cohort studies have suggested the presence of significant associations between midlife hypertension and the risks of late-life AD and VD. Metabolic syndrome has been shown to be a risk factor for the development of cognitive impairment, but no prospective cohort studies have investigated the significant influence of this syndrome on the risk of AD or VD. Some cohort studies have examined the associations of hypercholesterolemia with the risk of AD, but the results were inconsistent. Further studies are required to resolve these issues.