To clarify the influence of a westernized lifestyle on the risk factors for atherosclerosis and preclinical atherosclerosis in Japanese subjects, we surveyed a Japanese population and Japanese immigrants in the United States. Based on the extent of westernization of their lifestyle, the subjects were classified as Japanese (J), first generation Japanese-Americans (JA-I), and second or later generation Japanese-Americans (JA-II). The consumption of animal fat and simple carbohydrates increased in the order of J, JA-I, and J-II, while the subjects with strenuous physical activity decreased in the same order. The waist-hip ratio, fasting insulin level, serum cholesterol and triglyceride levels, and prevalence of hypertension increased in the same order as the dietary changes. The carotid intima-media wall thickness and the plaque size, which are indices of preclinical atherosclerosis, also increased in the order of J, JA-I, and JA-II. These data indicate that a westernized lifestyle aggravates the risk factors for atherosclerosis and influences the progression of preclinical atherosclerosis, in correspondence with the extent of westernization.