Background Mortality from ischemic heart disease (IHD) has been showing a marked decline in Japan. The objective of this study was to clarify the effects of age, time period, and cohort on mortality from IHD in Japan from 1955 to 2000. Methods and Results IHD death data from vital statistics and national population surveys were tabulated for 11 5-year age groups (from 30-34 to 80-84) and 10 quinquennial demographic profiles (from 1955 to 2000), to yield 20 5-year birth cohorts (mid-years from 1873 to 1968). The stratified Age-Period-Cohort model with 7 age classes shows: (1) mainly linear age trends for men with IHD, but effects below the linear age trends for women in middle-aged groups (40-74 years); (2) a decline in the effect of time period on IHD for both men and women from 1970 to 1990, and a sharp increase between 1990 and 1995 along with the change of the International Classification of Diseases Code; and (3) 2 patterns for both sexes for non-linear birth cohort effects, an increase (1873-1928) and a decrease (1928-1968) for men, and an increase (1873-1923) and a decrease (1923-1968) for women, with a stronger effect for women than for men born between 1893 and 1938, but the reverse for men and women born between 1938 and 1968. Conclusion Declining effects of birth cohort on mortality rates of IHD for young and middle-aged people suggest that mortality rates are likely to continue to decrease in Japan. (Circ J 2008; 72: 966 - 972)