Objectives To study the association between infection with chlamydia pneumonia and acute myocardial infarction (AMI). Methods Serology of chlamydia pneumoniae specific IgG、IgM antibodies were measured by microimmunofluorescence test in groups of acute myocardial infarction (AMI) and health control(HC). Results The total infection positive rates were 30.6% in HC group and 88.1% in AMI group, including the previous infection rates which were 30.6% and 71.4%, while the acute infection rates were 0% and 16.7%. The frequency of total infection, previous infection and acute infection was significantly higher in AMI group than in the HC group. Odds Ratio for the development of AMI were 16.82, 5.68, 14.2, respectively(95% CI 5.83 to 48.54,2.46 to 13.11, 1.68 to 119.97). Geometric mean IgG titre was significantly higher in patients with AMI compared with the HC group (P＜ 0.01). There is no IgM positive in HC group but there were two cases in AMI group. Conclusions The presence of high titers of immmunoglobulin G in AMI. Chlamydia pneumonia infection may be a risk factor for the AMI .