Comparisons of Hawaiian Japanese with Japanese living in Japan identify several differences between the 2 populations. The Hawaiian Japanese are heavier and taller; they consume more fat and protein; they have higher levels of serum cholesterol, more frequent fecal mutagens, and more frequent adenomatous polyps and diverticulae. These differences offer indirect support to the concept that the consumption of a Western diet favors the development of coronary heart disease and colon cancer which occur more frequently among the Japanese in Hawaii than in Japan. When assessed directly and prospectively, obesity, the serum cholesterol level, and dietary fat intake are positively associated with coronary heart disease. The serum cholesterol level and dietary fat intake are negatively associated with colon cancer, whereas the body mass index (height/weight) is positively related to this tumor in older men. The 2 diseases have shown dissimilar trends in the past 20 years, with coronary heart disease being stable at levels intermediate between the United States and Japan experience, whereas colon cancer has shown a steady increase with rates higher than those of whites in the United States. The differences in risk factors and trends displayed by the 2 diseases indicate that they affect different subsets of the westernized Japanese population. Additional studies are necessary if we are to establish the basis for these differences.