The association of Chlamydia pneumoniae with atherosclerosis of coronary and carotid arteries and the aorta has been demonstrated by seroepidemiology and by detection of the organism in atheromata. We investigated the frequency of C. pneumoniae seropositivity in patients with acute myocardial infarction (AMI). C. pneumoniae-specific antibodies were measured by the microimmunofluorescence test in 160 AMI patients and 160 control subjects matched for age and gender. The odds ratios (ORs) were 2.2 (95% confidence interval (CI), 1.2 to 3.9) for immunoglobulin (Ig)G and 2.7 (95% CI, 1.7 to 4.3) for IgA. After adjustment for other cardiovascular risk factors of age, gender, hypertension, diabetes, cigarette smoking and serum cholesterol, the ORs were essentially unchanged. This study confirmed that the observations of an association between antibody against C. pneumoniae and coronary heart disease in Western nations is also present in Japan. Our results are comparable to the previous seroepidemiological studies reporting ORs of 2.0 or greater.