Objectives: Whether triple antiplatelet therapy can inhibit enhanced platelet reactivity in AMI patients has not been determined. The aim of the study was to determine platelet inhibition by additive cilostazol to dual antiplatelet therapy in patients with acute myocardial infarction (AMI). Methods; AMI patients undergoing stenting were assigned to clopidogrel 75 mg/d (SMD, n =30), clopidogrel 150 mg/d (HMD, n =30) and adjunctive cilostazol 100 mg twice daily (TG,n =30). Platelet function was assessed by aggregometry and VerifyNow P2Y12 on and 30 days after discharge. Results: Baseline platelet measures were similar. Thirty days after discharge, percent inhibitions (PI) of maximal platelet aggregation (PA) with 20 ųM ADP were 5.8% in SMD, 16.1% in HMD, and 42.0% in TG (p < 0.001). PI of late PA with 20 ųM ADP were 10.3%, 32.9%, and 65.3%, respectively (p < 0.001). Similar results were shown with 5 ųM ADP and 6 ųg/ml collagen. Furthermore, PI of PRU were different (9.2% in SMD, 28.1% in HMD, and 41.6% in TG; p < 0.001). In terms of clopidogrel hyporesponsiveness (20 ųM ADP-induced maximal PA > 50%), fewer patients in TG (16.7%) met the definition as compared to those in SMD (80.0%) and HMD (60.0%) 30 days after discharge (p = 0.001). In TG, there were more consistent platelet inhibitions by all parameters as compared to HMD except for PI of PRU (p = 0.060). Conclusions: Among AMI patients undergoing stenting, triple antiplatelet therapy resulted in a greater antiplatelet effect than high MD clopidogrel.