Background and Objectives : Percutaneous coronary intervention(PCI) of unprotected left main coronary artery(LMCA) stenosis is promising in drug-eluting stent era. However, data are lacking as to the characteristics of the patient���s hemodynamics during LMCA stenting. The purpose of this study was to find which risk factor is affecting hemodynamic stability during LMCA stenting and whether hemodynamic support is needed or not according to the risk factors. Materials and Methods : From July 2003 to January 2006, 98 patients(Male=59, Femal=39), who visited Yeungnam university hospital, Keimyung University Dongsan hospital and InJe University Baik hospital in Busan, with angiographically evaluated unprotected LMCA stenosis were enrolled in this study. Group 1 (n=75) was defined as patients had hemodynamic changes(BP<90mmHg) during LMCA stenting. Group 2 (n=23) was defined as patients had no hemodynamic change during LMCA stenting. All patients were deployed stents on LMCA lesions without hemodynamic support and clinical, angiographical, procedural outcomes were compared between two group after procedure. Results : Baseline characteristics between two group were not different statistically. On the univariate analysis, Group2 had more patients diagnosed as MI than Group1 (39% vs 15%, P=0.017). Group 1 had more increased level of left ventricular(LV) ejection fraction than group 2(60짹10 vs 47짹11, p<0.05). In terms of lesion location in LMCA, group 2 had relatively more numbers of bifurcation lesion than group 1 ( 34% vs 78%, p<0.05). Group 2 needed more complex lesion techniques like kissing or crush stenting than group 1 ( 19% vs 48%, p<0.05). From the multivariate logistic regression analysis, presence of myocardial infarction (Odds Ratio(OR)=3.74, P=0.017), complex stenting procedure such as kissing or crushing (OR=3.99, P=0.007) bifurcation lesion (OR=4.58, P=0.004) and poor LV function (OR=9.95, P<0.001) were the independent predictors for the hemodynamic unstablility during LMCA stenting. Conclusion : The most important risk factor for hemodynamic unstability during LMCA stenting is LV function. We have to prepare for the hemodynamic support including IABP before procedure in this high risk patients.