Objectives: Reperfusion therapy for acute myocardial infarction (AMI) is indicated in the presence of ST elevation (STE) and ischemic symptoms. Previous MI may present with persistent STE or "left ventricular aneurysm" (LVA) morphology that mimics AMI. Hypothesis: A high ratio of T amplitude to QRS amplitude best distinguishes AMI from LVA. Methods: This was a retrospective cohort analysis. Patients with anatomical LVA by echocardiography were identified and those who presented to the ED with ischemic symptoms and STE of at least 1 mm in 2 consecutive leads and ruled out for acute left anterior descending coronary artery (LAD) occlusion were selected. Electrocardiograms (ECGs) were compared with a control group of 37 consecutive anterior AMI (aAMI) with proven acute LAD occlusion. Bundle-branch block was excluded. Various ECG measurements and ratios were compared. Results: Twenty patients with LVA met the inclusion criteria. The best discriminator was T amplitude sum to QRS amplitude sum ratio V1-V4, misclassifying only 4 (6.8%) of 59 cases at a cutoff of >0.22 for AMI. For aAMI and LVA, respectively, mean (±95% CI) ratio of the sum of T amplitudes in V1 to V4 to the sum of QRS amplitude in V1-V4 was 0.54 ± 0.085 and 0.16 ± 0.021 (P < .00012). Thirty-five of 37 aAMI had a ratio >0.22; the false negatives (ratio <0.22) had 11.5 and 6 hours of symptoms before the ECG. Twenty of 22 LVA had a ratio ≤0.22. Mean highest T/QRS ratio in V1-V4 was 1.1 ± 0.29 for an AMI and 0.26 ± 0.056 for LVA (P < 10 -7). Conclusion: T amplitude/QRS amplitude ratio best distinguishes aAMI from LVA in ECGs that meet STE criteria for reperfusion therapy. A high ratio is associated with an AMI.