Background: The aim of this study is to carify the effect of intracoronary (IC) administration of a bolus dose of abciximab during percutaneous coronary intervention (PCI) in the patient with acute ST segment elevation myocardial infarction(ASTEMI). Methods: We studied 42 patients with ASTEMI who underwent primary PCI on the targetvessel after administration of an IC abciximab bolus (0.25 mg/kg) prior to balloon inflation (IC Group), and 44 control patients who were treated by intravenous (IV) abciximab (IV Group). In-hospital major adverse cardiac event (MACE; death, myocardial infarction, urgent revascularization) and corrected TIMI frame count (CTFC) was assessed. CTFC was assessed before abciximab treatment and the end of PCI. Results: There were no differences between the groups with regard to baseline clinical, and angiographic characteristics and the success rate of intervention. After administration of abciximab, CTFC significantly decreased from 48.6짹37.4 to 20.1짹18.5 in the culprit vessel of IC group, while 49.8짹42.6 to 28.9짹31 in the IV group (P=0.02). The incidence of in-hospital MACE was not significantly different between the two groups. The incidence of bleeding complication showed a trend toward reduction in IC Group (4.8%) compared with IV Group (6.8%). Conclusions: The IC administration of abciximab bolus acutely decreases CTFC through culprit vessels of patients with ASTEMI undergoing primary PCI. Further large scale studies are warranted to evaluate the potential clinical benefits associated with IC abciximab administration.