Objectives The aim of this pilot study was to assess the degree of platelet inhibition by cilostazol in patients with CYP2C19 mutant allele. Background Cilostazol not only inhibits platelet aggregation, but also has beneficial effects on oxidative stress and endothelium. Antiplatelet response to clopidogrel is mainly influenced by carriage of CYP2C19 mutant allele, whereas antiplatelet response to cilostazol is largely determined according to the activity of the CYP3A4 pathway. Methods A total of 24 patients with CYP2C19 mutant allele were enrolled. They were treated with coronary stent and on chronic dual antiplatelet therapy (aspirin 100-mg/day and clopidogrel 75-mg/day ��� 6 months). Clopidogrel was switched with cilostazol 100-mg twice daily for 14 days. Platelet reactivity was measured before switch and at 14 days after switch using conventional aggregometry and VerifyNow assay. Primary endpoint measure was maximal platelet aggregation (Aggmax). Results Aggmax values after the addition of 5 and 20 關mol/l ADP did not differ after clopidogrel versus cilostazol treatment (44.1 짹 15.8% vs. 44.8 짹 17.3%, p = 0.842, and 57.6 짹 14.4% versus 59.8 짹 17.5%, p = 0.561, respectively). Values of late platelet aggregation and disaggregation were similar after clopidogrel versus cilostazol treatment. Likewise, values of P2Y12 reaction unit and % inhibition after clopidogrel versus cilostazol were not different (254.4 짹 62.8 vs. 259.1 짹 79.8, p = 0.729, and 22.7 짹 12.7% versus 21.3 짹 18.9%, p = 0.699, respectively). Conclusions In carriers of CYP2C19 mutant allele, adjunctive cilostazol on aspirin treatment can achieve similar inhibition of ADP-induced platelet aggregation, as compared with standard dual antiplatelet therapy with clopidogrel and aspirin.