Background and Purpose :There is growing evidence linking Chlamydophila pneumoniae infection to coronary heart disease and atherosclerosis. Association of C. pneumoniae with coronary heart disease was evaluated Methods : 53 cases of acute coronary syndrome (ACS), 27 cases of old MI and 61 normal controls were enrolled. Anti-C. pneumoniae IgG and IgM antibodies was measured using enzyme-linked immunosorbent assay (ELISA) and microimmunofluorescence (mIF) and C. pneumoniae DNA was detected using polymerase chain reaction (PCR) . Among those with ACS and OMI ( with coronary heart disease ), 58 cases had coronary angiography and angiographic scores were calculated. Results : Seropositivity of anti-C. pneumoniae IgM antibody by ELISA was 9.8% in control group, 11.0% in OMI and 17.0% in ACS group (P=0.001), respectively. Seropositivity of anti-C. pneumoniae IgM antibody by mIF (Fuller Lab, Cal, USA: mIF-F) was 9.8% in control group, 37.0% in OMI and 32.1% in ACS group ( P=0.004), respectively. Seropositivity of anti-C. pneumoniae IgM antibody by mIF (home-made: mIF-H) was 4.9% in control group, 18.5% in OMI and 25.0% in ACS group ( P<0.001), respectively. Seropositivity of anti-C. pneumoniae IgG antibody by ELISA was 59.0% in control group, 85.2% in OMI and 86.8% in ACS group (P=0.001), respectively. Seropositivity of anti-C. pneumoniae IgG antibody by mIF-H was in 60.7% in control group, 85.2% in OMI and 92.3% (P=0.001) in ACS, respectively. Vessel scores, stenosis scores and extent scores were not statistically different between seropositive and negative groups. Conclusions : Although seropositivity of C. pneumonia was much higher in ACS and old MI, it was not related to coronary angiographic severity.