Chlamydia pneumoniae can possibly trigger and maintain inflammation in coronary arteries. Chlamydia pneumoniae DNA and chlamydial lipopolysaccharide (cLPS) were measured 3 times during a 1-y period in 97 patients with acute coronary syndrome. Chlamydia pneumoniae DNA in peripheral blood mononuclear cells was detected in 8 (8.2%) patients at the initial hospitalization and in 9 (10.6%) patients at 3 months. One y after the acute coronary syndrome, Chlamydia pneumoniae DNA was not found in any patients. Serum cLPS levels were elevated at inclusion, and declined significantly during follow-up (1.40 microg/ml; (0.20-2.91), median; (range of 25th to 75th percentiles) at inclusion, 0.44 microg/ml; (0.00-1.39) at 1 y; ANOVA p<0.0001). cLPS levels correlated significantly to Chlamydia pneumoniae DNA positivity at 3 months (p=0.003). In conclusion, Chlamydia pneumoniae DNA is present during acute coronary syndrome and in the recovery period, but declines in stable state, suggesting a role of the bacterium in the acute phase of coronary syndrome.