Background: Previous studies have shown that cilostazol may not only prevent subacute stent thrombosis, but also may have positive effect in the prevention of restenosis. However, the effect of cilostazol on restenosis after successful deployment of drug-eluting stent (DES) in patients with diabetes mellitus was not evaluated. Methods and Results: A total of 320 patients at 8 clinical sites will be randomized. At present 202 patients (60.5짹10.4 years old, 113 males) who underwent successful stenting were randomized to aspirin and cilostazol (group I, n=99, 60.3짹10.6 years old) vs. aspirin and clopidogrel (group II, n=103, 62.1짹10.0 years old) after one month of aspirin, cilostazol, and clopidogrel combination treatment. Baseline patient characteristics were not different between the two groups. The types of DES implanted were not different between the groups (26 Cypher and 72 Taxus in group I vs. 40 Cypher and 64 Taxus in group II, p=ns). There were no differences in angiographic and procedural characteristics between the groups. Major adverse cardiac events (MACE) including acute and subacute stent thrombosis within one month were not developed in both groups. Sixty one patients (26 patients in group I, 35 patients in group II) performed follow-up coronary angiogram. The rate of angiographic restenosis at the 6 month was 3.8% (0% in group I vs. 3 (8.6%) in group II, p=0.25). MACE at 6 month were not different between the groups (1 death in group I vs. 3 target lesion revascularization in group II). Conclusion: Our results demonstrated that the effects of combination therapy with aspirin and cilostazol for the prevention of stent thrombosis and restenosis were comparable to those of aspirin and clopidogrel.