Background The Japan Arteriosclerosis Longitudinal Study-Existing Cohorts Combine (JALS-ECC) is a pooled study based on individual participant data from existing prospective cohort studies in Japan. Its purpose was to consider associations between risk factors and cardiovascular disease (CVD) outcomes, as well as differences between subgroups, defined by age, gender or geographical region, which could not be detected in the smaller samples. Methods and Results Individual records for 66,691 participants in 21 cohort studies were pooled, accounting for a total of 575,628 person-years. From this data, there were 409 deaths attributed to stroke and 169 deaths attributed to coronary heart disease (CHD). Total stroke and CHD events were 1,478 and 178, respectively. Of the 1,424 total stroke events with a reported stroke subtype, 975 were classified as ischemic, 267 as hemorrhagic, and 178 as subarachnoid hemorrhage. Conclusion The JALS-ECC collected data from existing cohort studies covering a diverse Japanese population, which has provided information about the effects of modifiable factors on the risks of the CVD. Such information should provide a reliable basis for establishing prevention strategies. (Circ J 2008; 72: 1563 - 1568)