Background and Purpose : $Clopidogrel(Plavix^{circledR})$, a new thienopyridine derivative similar to ticlopidine, is an inhibitor of platelet aggregation induced by adenosine diphosphate(ADP). There has been no comparative study between clopidogrel and aspirin on its antiplatelet aggregatory effect and side effects in Korea. The aim of our study was to evaluate the efficacy and the side effect of clopidogrel in patients with acute ischemic stroke. Methods : Clopidogrel(75mg once daily) and aspirin(300mg once daily) were administered for 4 weeks in 4.0 patients, who suffered from acute ischemic stroke. Their efficacy was evaluated by platelet aggregation test and the recurrence of ischemic event at before and after the medication schedule. The efficacy and the incidence of clinical and laboratory side effects were compared in 4 weeks after the administration of the drugs. Results : Platelet aggregation induced by ADP, collagen and epinephrine was significantly inhibited in both clopidogrel and aspirin groups. There was no significant difference of platelet aggregation effect between two groups and there was no recurrence of ischemic event. The side effects induced by clopidogrel were dyspepsia(4.5%), gastric discomfort(4.5%) and abnormal LFT(9.0%), and the incidence of side effects was lower in clopidogrel group(18%) than aspirin group(26%). Conclusion : The anti-platelet effect of clopidogrel was comparable to that of aspirin, and the adverse effect of clopidogrel was lower than that of aspirin. Thus clopidogrel is as effective as aspirin and safe drug for acute ischemic stroke. Considering long term medication is essential for the treatment of arteriothrombotic diseases, we would say that clopidogrel is effective as well as safe.