Background and Objectives : There is serological and epidemiological evidence of an association between Chlamydia pneumoniae infection and coronary artery disease. We conducted a randomized study using azithromycin treatment in Korean patients with acute coronary syndrome (ACS) underwent percutaneous coronary intervention (PCI) to determine whether azithromycin treatment reduce inflammatory markers and major adverse cardiac events (MACE). Methods: One hundred twenty nine patients were randomly allocated to receive 500 mg azithromycin daily for 3 days before and after PCI, followed by 500mg/week for 2 weeks (Group I: 64 patients, 43 male, 60.0짹10.0 years) or placebo (Group II: 65 patients, 45 male, 59.6짹10.1 years). Patients were followed up for 12 months. The primary endpoints were cardiac death, recurrent myocardial infarction (MI), target lesion revascularization (TLR) and Non-TLR during 12-month follow-up. Results: There were no differences between the two groups in baseline characteristics, coronary angiography finding, lesion characteristics, baseline inflammatory marker, and baseline antibody titers of Chlamydia pneumoniae, Helicobacter pyroli and Mycoplasma. After the antibiotics treatment, ESR and CRP decreased from 19.6짹20.7 mg/dL and 0.75짹0.99mg/dL to 9.36짹10.5 mg/dL and 0.22짹0.20 mg/dL in group I (p=0.002, 0.001 respectively), and from 19.6짹21.5 mg/dL, 1.44짹2.69mg/dL to 10.4짹10.8 mg/dL, 0.55짹1.48 mg/dL in group II (p=0.052, <0.0001 respectively). However, there were no significant changes in serologic markers. MACE developed in 17 (26.6%) out of 64 patients in group I: 2 death, 2 MI and 13 TLR, and in group II, 14(21.5%) out of 65 patients had MACE: 2 death, 1 MI and 10 TLR during 12-month clinical follow-up (p=NS). Conclusions: Short-term treatment with azithromycin does not reduce the cardiac events in Korean patients with ACS after PCI.