Background: Cilostazol, a phosphodiesterase III inhibitor, has been known to reduce smooth muscle proliferation, intimal hyperplasia, and coronary restenosis. We compared the effects of cilostazol on carotid intima-media thickness (IMT) and clinical events during the 12 months follow-up in patients with acute coronary syndrome. Methods: Longitudinal images of left and right carotid IMT were measured with the use of 10MHz linear vascular probe in 130 patients with acute coronary syndrome. After coronary intervention, patients were randomly assigned to either cilostazol 100 mg BID (n=64) or placebo (n=66) for 12 months. All patients received aspirin and clopidogrel. Clinical events such as stroke, death, myocardial infarction, and target lesion revascularization (TLR) were compared during the follow-up. Results: The carotid IMT was significantly reduced in the Cilostazol Group (right: 0.75 짹 0.45 vs 0.66 짹 0.39 mm, p<0.05; left: 0.70 짹 0.48 vs 0.62 짹 0.33 mm, p<0.05) compared with the Placebo Group (right: 0.78 짹 0.52 vs 0.77 짹 0.69 mm, p=0.74; left: 0.72 짹 0.41 vs 0.74 짹 0.58 mm, p=0.81). The level of C-reactive protein was significantly reduced in the Cilostazol Group (1.83 짹 1.33 vs 1.02 짹 0.89 mg/L, p<0.05) compared with the Placebo Group (1.69 짹 1.49 vs 1.45 짹 1.44 mg/L, p=0.67). Clinical events such as stroke (1 patient in the Cilostazol vs 1 patient in the Placebo, p=0.86), death (1 patient in the Cilostazol vs 1 patient in the Placebo, p=0.86), myocardial infarction (1 patient in the Cilostazol vs 2 patients in the Placebo, p=0.65), and TLR (1 patient in the Cilostazol vs 3 patients in the Placebo, p=0.23) showed no significant differences during the follow-up. Conclusion: Cilostazol therapy in addition to aspirin, clopidogrel and statin reduced carotid IMT in patients with acute coronary syndrome. However, the rate of clinical events revealed no significant differences between the 2 groups.