Background: Glycoprotein IIb/IIIa receptor blocker is useful tool to inhibit platelet activation during percutaneous coronary intervention (PCI). However, its optimal inhibiton has not been evaluated in oriental patients. We evaluated the acute inhibitoy effect of tirofiban on the platelet function. Methods: Since November 2002 to July 2003, 75 patients (mean ages, 59 짹 11 years [range, 28-77], 53 male) were enrolled in this study. Tirofiban was administrated by bolus of 0.15 mg/kg, followed by continuous infusion of 0.125 關g/kg/min. Blood sampling was done to estimate platelet function using a Ultegra Accumetric device before and 15 min, 1 hour after tirofiban infusion during PCI. Results: Mean percentage of platelet inhibiton after 15 min and 1 hour of tirofiban infusion were 86.7 짹 7.8% and 82.2 짹 9.1%, each. More than 90% of platelet inhibition was achieved in 41% and 19% after 15 min and 1 hour after the drug infusion. More than 80% of platelet inhibition was achieved in 78% and 46%. Mean pre and post activated clotting time (ACT) was increased from 248 짹 71 to 371 짹 107 sec and platelet count was not changed after the procedures (226,000 to 224,000). Conclusions: With current dose of tirofiban, optimal suppression of the acute platelet inhibition during PCI was achieved less than half of the patients (criteria of >90% inhibiton). Further studies are needed to confirm this data.