Introduction: Several predisposing and risk factors are introduced as main causes of coronaryatherosclerosis which is the main cause of myocardial infarction (MI). In recent years, chronic andpersistent infections are considered as such factors. This study is basically on determination ofseropositivity to Chlamydia pneumonia to reveal the role of acute and chronic infections due to thesebacteria as a risk factor for MI.Materials and Methods: 273 serum samples from MI patients and 109 samples from control groupwere examined by using commercial quantitative ELISA kits to measure specific anti chlamydiapneumonia antibodies (IgM and IgG). Two groups were matched for age and sex.Results: IgM titers (ELISA) were negative in all patients and control cases, indicating lack of acuteChlamydial infection, but IgG titers were positive in 273 patients (94.4%), (mean average titer: 108RU/ml) and in 109 control cases (84.4%) (mean average of titer: 61.9 RU/ml). We found nosignificant relationship between seropositivity to chlamydia pneumonia antibodies (IgG) with MI(P=0.104).Conclusion: In this study, no significant relationship was observed between serpositivity toChlamydia pneumonia and subsequent incidence of MI. It seems that a large scale serological studywith a diagnostic molecular methods for detection of genome in biopsy tissue of atheroscleroticcoronary artery will be more helpful to reveal the expected relationship.