Background Obesity is an important risk factor for the occurrence of coronary artery disease (CAD) in Western countries and furthermore, it often coexists with other CAD risk factors such as hypertension, dyslipidemia and diabetes mellitus. However, it is uncertain whether obesity is a CAD risk factor in Japan because Japanese are relatively thin on average. Methods and Results The CAD risk associated with obesity (body mass index ≥25.0) 10 years before as well as at the time of the survey was assessed in a case - control study of acute myocardial infarction (AMI), which compared 660 AMI patients aged 40-79 years and 1,277 community controls, matched to each case by sex, year of birth, and residence. The prevalence of current obesity did not show any material difference between cases and controls, but compared with controls, past obesity was much more frequent in cases. Even after controlling for other CAD risk factors, past obesity was associated with a 2-fold increase in the risk of AMI. Past obesity was associated with an increased risk of AMI even without current obesity. Conclusion Past obesity is a CAD risk, even after weight reduction. (Circ J 2004; 68: 41 - 46)