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Despite certain clinical benefit in using clopidogrel in patients undergoing percutaneous coronary intervention (PCI), some patients do not attain adequate antiplatelet effects. In this study, the authors investigated the response to clopidogrel in Iranian patients after PCI. Patients who were candidates for elective PCI were enrolled in this study. All patients had received aspirin 80 to 325 mg daily for ≥1 week before PCI. Blood samples were taken from patients at baseline, 2 hours after taking a 600-mg loading dose of clopidogrel, and 24 hours and 30 days after stenting. Platelet aggregation was measured by light transmittance aggregometry with adenosine diphosphate (5 and 20 μM) and arachidonic acid (500 and 5000 μg/mL). One hundred twelve patients were included (79 men, 33 women). Maximal and minimal clopidogrel nonresponsiveness occurred at 2 hours (26%) and 48 hours (13%) after taking 600 mg clopidogrel, respectively. Pretreatment platelet reactivity had no effects on posttreatment platelet reactivity. Moreover, clopidogrel responsiveness did not correlate with pretreatment reactivity. Patients' demographic and procedural characteristics had no significant effect on clopidogrel responsiveness. The frequency of clopidogrel nonresponsiveness in this study was similar to other studies. However, clopidogrel required more than 2 hours for induction of its maximal antiplatelet effect in this study.