Background: Clopidogrel has become a standard treatment after urgent percutaneous coronary revascularization. Due to its enhanced and irreversible platelet inhibition, however, patients undergoing urgent surgical revascularization have a higher risk of bleeding complications and transfusion requirements. We therefore evaluated the effect of preoperative continuous administration of clopidogrel on the incidence of hemorrhagic complications in patients undergoing off-pump coronary artery bypass surgery with acute coronary syndrome. Methods: From March 2004 to September 2006, 172 patients with acute coronary syndrome underwent isolated off-pump coronary artery bypass surgery; of these patients, 70 (40.7%) did and 102 (59.3%) did not take clopidogrel before surgery. Seventy patients in each group were matched using propensity scores, and the associations between preoperative continuous administration of clopidogrel and postoperative bleeding, hemostatic reoperation, blood products received, the need for multiple transfusions, and early graft patency by coronary computed tomography were assessed. Results: Univariate analysis showed that the continuous clopidogrel group had similar postoperative bleeding for 24 hours (601.4짹312.6 mL vs.637.2짹452.4 mL, p=0.616) and similar rates of reexploration (1.4% vs.1.4%), perioperative blood transfusion (33.3% vs.34.3%, p > 0.05), and platelet transfusion (2.9% vs.7.1%, p=0.44) compared with the non-continuous group. Conclusions: Preoperative continuous administration of clopidogrel did not increase the risk of hemorrhagic complications in patients with acute coronary syndrome undergoing isolated off-pump coronary artery bypass surgery. These findings indicate that surgery for the patients with acute coronary syndrome after clopidogrel treatment should not be delayed until platelet function returns because these patients may be exposed to the risk of recurrent myocardial ischemic events.