This paper reports the results of the largest autopsy-based comparative study of atherosclerotic lesions between young Japanese and Americans, aged 15-34 years and autopsied between 1987 and 1995, by analyzing the data from the Japanese second nation-wide study of atherosclerosis and Pathobiological Determinants of Atherosclerosis in Youth study in the USA. In the right coronary arteries, in Japanese, fatty streaks were well established in the second decade of life with very little increase in the remaining age groups up to age of 34 years. In contrast, in American subjects, the average percentage of surface involvement of fatty streaks and raised lesions proceeded steadily with age without an obvious plateau throughout the 20-year period. The extent and prevalence of the raised lesions was much greater in Americans than in Japanese in 25-29 and 30-34-year age groups. Moreover, the rate of progression of raised lesions was much more rapid in Americans. These findings are in keeping with the fact that coronary heart disease (CHD) death rates are much higher in the USA than in Japan. In light of data showing that the risk factor profiles for CHD have become very similar between the two countries, these differences need to be explained.