Background: Prompt restoration of blood flow in the infarct-related artery is essential to myocardial salvage and mortality reduction in patients with total occluded coronary artery. As the benefits of reperfusion decline rapidly with time, initial diagnostic tool is very important. In Korea, 12-lead surface electrocardiogram (ECG) is a routine diagnostic tool to detect acute myocardial infarction (AMI). However, 12 lead ECG was not suitable for diagnosing posterior wall myocardial infarction (MI) associated with left circumflex artery (LCX). This study was designed to diagnostic uncertainty of 12 lead ECG for diagnosing posterior wall MI. Methods: Total 2,281 patients (1,683 males, 59.7짹13.3 year-old) showing total occlusion (pre-TIMI flow: 0) in single, de novo native coronary arteries on angiography were enrolled from Nov 2005 to Jan 2008 in KAMIR. These patients were divided into the three groups according to coronary artery (Left anterior descending artery (LAD), Right coronary artery (RCA), LCX. Results: There was no significant difference in clinical characteristics. But, in 12 lead ECG, V1-V4 ST elevation was frequently associated with LAD group (78.9% vs 6.0 vs 5.0%, p<0.001), II III aVF ST elevation : RCA group (75.8% vs 5.8% vs 26.8%, p<0.001), I aVL, V5-V6 ST elevation : LCX group (18.4% vs 10.5% vs 3.9%, p<0.001). However, in initial diagnosis, ST elevation myocardial infarction (STEMI) was frequently diagnosed in LAD, RCA group by 12 lead ECG compared with LCX group (87.0% vs. 82.3% vs. 46.3%, p<0.001). Subsequently, the rate of primary PCI was higher in LAD, RCA group (78.9% vs. 74.5% vs. 43.4%, p<0.001). Door to balloon time lesser than 90 minutes was more associated with LAD and RCA group compared with LCX (52.8% vs 51.0% vs 31.3%, p<0.001). Conclusions: 12 lead ECG is not effective diagnostic tool for detect LCX total occlusion. If STEMI in LCX territory was suspected clinically, other method such as 18-lead ECG should be considered.