BACKGROUND: Evidences support the possible involvement of microorganisms such as Chlamydia pneumonia in the pathogenesis of ischemic heart diseases through a chronic inflammatory process. The aim of this study was to determine the relation between Chlamydia pneumoniae seropositivity with acute myocardial infarction and its related risk factors. METHODS: In this case-control study, 88 patients admitted in CCU with a diagnosis of acute coronary syndrome, without a history of chronic diseases including cancers were selected as cases and 49 surgical patients without an evidence of cardiovascular disease according to clinical examinations and ECG were selected as controls. Demographic characteristics and background risk factors were obtained using a questionnaire by expert nurses. Venous blood sample was obtained from participants for measuring the anti Chlamydia IgG and IgM antibodies using ELISA method. The prevalence of antibodies was compared in both groups and its relation with coronary syndrome was evaluated. RESULTS: 88 and 49 patients were enrolled in case and control groups, respectively. Mean age of patients and the controls was 14 ± 59.7 and 13 ± 56.9 years, respectively (P = 0.26). Anti Chlamydia IgG seropositivity rate was 63(71.9%) and 23(46.9%) in case and controlcontrol groups, respectively (P < 0.01; OR: 2.85; CI 95%: 1.38 - 5.9). Anti Chlamydia IgM was positive in 1 patient and 1 control. Anti Chlamydia IgG seropositivity rate was higher in patients older than 50 years old than those younger than 50 years old (OR: 2.83; CI 95%: 1.31 -1.14). There was a significant relation between BMI, smoking and Anti Chlamydia IgG seropositivity. CONCLUSION: Considering the relation between anti Chlamydia antibody IgG seropositivity with BMI and myocardial infarction, it seems that appropriate diagnosis and treatment of these prone patients can be benefical.