Vascular dementia (VD) is more prevalent than Alzheimer's disease (AD) in Japan, while AD is more common in Western countries. In the Hisayama study, a community-based cohort study of Japan, the prevalence of VD decreased in men during the 7-years (1985-1992) follow-up period, while the prevalence of AD remained unchanged both in men and women. The incidence of dementia increases with age, particularly AD aged 85 or older. Hypertension is a major risk factor for VD. Other risk factors include age, prior stroke, diabetes, alcohol intake, heart disease, and smoking. In contrast, age, a family history of dementia, a low educational level, and low physical activity are risk factors for AD. The role of hypertension in AD remains controversial; there has been positive, negative, or no association existed between blood pressure levels and AD. A recent clinical trial has disclosed the potential preventive effect of antihypertensive treatment on the incidence of dementia, especially of AD. Although the role of vascular factors for the pathogenesis of AD is becoming recognized, the effectiveness of antihypertensive treatment on the prevention of AD should be further clarified in the future studies.