Background: Various factors affect on clinical outcomes in patients with ST segment elevation myocardial infarction (STEMI). The aim of the present study was to compare the long term clinical outcomes according to inserted stent number in the culprit vessel during primary percutaneous coronary intervention (PCI). Method: Using data from Korea Acute Myocardial Infarction Registry (KAMIR; November 2005-December 2006) and Korea Working Group on Myocardial Infarction (KorMI; January 2007-January 2008), a total of 1668 patients presented with STEMI who proven to have single vessel disease and underwent primary PCI were analyzed. We compared 12-month MACEs (cardiac or non-cardiac death, re-myocardial infarction, target lesion or target vessel revascularization, coronary artery bypass graft and stent thrombosis) in patients with single stent group (n=1391, 83.4%) to those with multiple stents group (n=277, 16.6%). Results: The cumulative incidence of 12-month MACEs was significantly higher in the multiple stent group than single stent group (10.4% vs 16.7%, p=0.010), which was mainly due to increased target lesion revascularization (3.9% vs 9.4%, p=0.001). By multivariate analysis, multiple stent insertion was identified to be significantly associated with increased MACEs at 1 year (OR=1.590, 95% confidence interval: 1.028, 2.459, p=0.037). conclusion: This study showed the number of inserted stents in STEMI during primary PCI is an independent predictor of clinical outcomes.