Aim To study the association between prior infection with chlamydia pneumoniae and chronic coronary heart disease. Methods Chlamydia pneumoniae-specific lgG and lgM antibodies were measured in 45 patients with acute myocardial infarction (AMI), 32 patients with old myocardial infarction (OMI) and 101 control by microimmunofluorescence test. Results The positive rate of specific lgG antibodies was 95.6% (43/45) in patients with AMI, 81.3% (26/32) in patients with OMI and 56.4% (57/101) in controls, the geometric mean titres of spesifc lgG antibodies were 95.6±113.5, 51.7±39.1 and 35.3±33.7 respectively. The positive rate and geometric mean titres of specific lgG antibodies in patients with AMI and OMI were significantly higher than those in controls. Conclusion There may exist an association between chronic chlamydia pneumoniae infection and coronary heart disease.