Background – Although addition of cilostazol to dual antiplatelet therapy results in greater platelet inhibition, it is unknown whether adjunctive cilostazol can reduce the rate of clopidogrel hyporesponsiveness (CH). We performed the study to determine the impact of adjunctive cilostazol on platelet inhibition in patients with CH undergoing stenting. Methods and Results - Sixty patients with CH were assessed for platelet reactivity after 300 mg loading of clopidogrel. CH was defined as baseline maximal platelet aggregation (MPA) > 50% with 5 ųM ADP. Patients were assigned to either adjunctive cilostazol 100 mg twice daily (CIL; n =30) or high maintenance-dose (MD) clopidogrel 150 mg/d (HMD; n = 30). Platelet function was assessed at baseline and after 30 days by transmittance aggregometry and VerifyNow P2Y12. Both baseline platelet function were similar. After 30 days, fewer patients in CIL as compared to HMD met the definition of CH (3.3% vs. 26.7%, P = 0.012). Percent inhibition (PI) of MPA with 5 ųM ADP was greater in CIL than in HMD (51.8 짹 21.0% vs. 28.0 짹 18.5%, P < 0.001). PI of late PA with 5 ųM ADP was also higher in CIL than in HMD (70.9 짹 27.3% vs. 45.3 짹 23.4%, P < 0.001). PI of PA with 20 ųM ADP in CIL was consistently greater than that in HMD. PI of PRU demonstrated a greater antiplatelet effect in CIL than in HMD (39.7 짹 23.8% vs. 23.5 짹 30.2%, P = 0.025). Conclusion - Adjunctive cilostazol reduces the rate of CH and intensifies platelet inhibition as compared to high MD clopidogrel.