A significant proportion of patients with myocardial infarction are missed upon initial presentation to the emergency department. The 12-lead electrocardiogram (ECG) has a low sensitivity for the detection of acute myocardial infarction, especially if the culprit lesion is in the left circumflex artery (LCA). This study was designed to evaluate the benefit of adding 3 posterior chest leads on top of the 12-lead ECG to detect ischemia resulting from LC disease, using a model of temporary balloon occlusion to produce ischemia. We studied 53 consecutive patients who underwent clinically indicated coronary interventions. At the time of coronary angiography, the balloon was inflated to produce complete occlusion of the proximal LCA. We recorded and analyzed the changes noted on the 15-lead ECG, which included 3 posterior leads in addition to the standard 12 leads. In response to acute occlusion of the LCA, the posterior chest leads showed more ST elevation than the other leads, and more patients had ST elevation in the posterior leads than in any other lead. The 15-lead ECG was able to detect ≥0.5 mm (74% vs 38%, p <0.0001) and ≥1 mm (62% vs 34%, p <0.0001) ST elevation in any 2 contiguous leads more frequently than the 12-lead ECG. In conclusion, the 15-lead ECG identified more patients with posterior myocardial wall ischemia because of temporary balloon occlusion of the LC than the 12-lead ECG. This information may enhance the detection of posterior MI in the emergency department and potentially facilitate early institution of reperfusion therapy. © 2009 Elsevier Inc. All rights reserved.