Background: Although cilostazol has been reported to reduce neointimal hyperplasia and restenosis after bare–metal stent (BMS) implantation, it is not known if this effect occurs after drug-eluting stent (DES) implantation in patients with diabetes mellitus (DM). Methods: This randomized, multicenter, prospective study compared triple antiplatelet therapy (aspirin, clopidogrel and cilostazol, triple group, n=200) and dual antiplatelet therapy (aspirin and clopidogrel, standard group, n=200) for 6 months in patients with DM receiving DES. The primary end point was in–stent late loss at 6–month angiography according to the intention–to–treat principle. Results: The two groups had similar baseline clinical and angiographic characteristics. The in–stent (0.25짹0.53 mm vs. 0.38짹0.54 mm, p=0.025) and in-segment (0.42짹0.50 mm vs. 0.53짹0.49 mm, p=0.031) late loss were significantly lower in the triple versus standard group, as were the 6-month in-segment restenosis (8.0% vs. 15.6%, p=0.033) and 9-month target lesion revascularization (TLR) (2.5% vs. 7.0%, p=0.034) rates. At 9 months, major adverse cardiac events (MACE) including death, myocardial infarction, and TLR, tended to be lower in the triple than in the standard group (3.0% vs. 7.0%, p=0.066). Subgroup analysis showed that triple plus SES patients had a significantly lower in-segment restenosis (0%, 0/88) rate than standard plus SES (8%, 7/88, p=0.014), triple plus PES (17.3% 13/75, p<0.001), and standard plus PES (24.1%, 19/79, p<0.001) patients. Multivariate analysis showed that, SES and the use of cilostazol were strong predictors of reduced restenosis and TLR. Conclusiona: Patients with DM receiving triple antiplatelet therapy after DES implantation had decreased angiographic restenosis and extent of late luminal loss, resulting in reduced risk of 9-month TLR, compared to patients receiving dual antiplatelet therapy.