Background: In the context of outpatients planned for elective percutaneous coronary intervention(PCI), the issue of optimal loading dose and time of clopidogrel administration remains controversial. The present study evaluated potential differences among 3 clopidogrel loading doses with regard to periprocedural ischemic events, bleeding complications, and 30-day major adverse cardiac events(MACEs).Methods: A total of 140 patients (72 men, 63.3짹9.3 years) with stable angina, low or intermediate unstable angina, or silent ischemia were randomly assigned to each 3 groups before PCI; clopidogrel 300mg loading 6hr before PCI(group A, n= 31), 450mg loading just before PCI(group B, n=45), and 600mg loading just before PCI(group C, n=64). Results: Baseline clinical and angiographic characteristics were similar among 3 groups. There were no significant differences among 3 groups in postprocedural CK-MB increase higher than > 2��UNL( A vs B, p= 0.815, A vs C, p=0.481). Significant decrease of hematocret(>15points) was not shown in all 3 groups and 30-day MACEs occurred only one case in group A. Conclusion: In patients eligible to outpatient catheterization, 450mg and 600mg clopidogrel loading dose given immediately before PCI is sufficient compared with conventional 300mg loading 6hr before PCI. According to this study, 450mg loading dose immediately before PCI is safe, convenient and alternative method, although 600mg loading dose is also safe but it was not associated with fewer periprocedural events and improved 30-day outcomes compared with 450mg.