Alzheimer's disease (AD) is the most frequent type of dementia in old age. The pathology of AD is not well understood. One hypothesis states that AD has a vascular basis. This hypothesis is controversial because AD has traditionally been studied and treated as a separate disease from vascular dementia. This article reviews the epidemiologic evidence supporting an association between AD and two major vascular risk factors-blood pressure and diabetes. Both methodologic and biologic explanations for these associations are discussed.