It has been observed that among men of Japanese ancestry, there is a gradient in CHD mortality increasing from Japan to Hawaii to California. A study of 11,900 Japanese men in Hiroshima and Nagasaki, Japan, Honolulu, Hawaii, and the San Francisco Bay Area of California has been conducted to investigate this disease difference. This paper describes the selection of the study populations and their age distributions, and outlines the study methods. This paper also introduces and briefly summarizes four papers that give the results for mortality comparisons, biochemical and blood pressure distributions and results for heart disease prevalence among the three cohorts.