Background: There is serological and epidemiological evidence of an association between Chlamydia penumoniae infection and coronary artery disease. We conducted an randamized study using azithromycin treatment against these bacteria in patients with acute coronary syndrome (ACS) underwent coronary stenting to determine whether azithromycin treatment reduce inflammatory markers and major adverse cardiac events (MACE). Methods: We assessed the effect of azithromycin in a randamized study in 140 patients with ACS underwent stenting. Patients were randomly allocated to receive 500 mg azithromycin daily for 3 days before and after of PCI, followed by 500mg week for 2 weeks or placebo. Patients were followed up for 3 months. The primary endpoints were cardiac death, recurrent myocardial infarction (MI), target vessel revascualrization (TVR). Results: There is no difference between the two groups in baseline characteristics, coronary angiography finding, lesion characteristics, baseline inflammatory marker, baseline antibodies of Chlamydia penumoniae, Helicobacter pyroli, Mycoplasma. After the antibiotics treatment, ESR and CRP decreased from 17.2짹21.5 mg/dl, 0.83짹1.00mg/dl to 8.7짹7.10 mg/dl, 0.23짹0.24 mg/dl (p<0.05), but not showed significant change of infection markers in the azithromycin group. In 98.6% patients, clinical follow-up was done during 3 month(3-10 months). Of the 70 patients in the azithromycin group, 1(1.4%) died, 2(1.4%) developed MI, and 17(24.3%) had recurrent TVR. In the 70 placebo group the corresponding numbers of patients were 1(1.4%), 1(1.4%), and 15(21.4%), respectively (p=0.841). There was no significant difference in MACE between the two groups. Conclusions: Short-term treatment with azithromycin does not reduce presenting the cardiac events in patients with ACS underwent coronary stenting.