We developed a scoring system to predict the artery responsible for an acute myocardial infarction (AMI) using ST-segment and T-wave changes on the initial electrocardiogram (ECG) using data from 228 patients (development set) with symptoms compatible with AMI and tested in a similar group of 223 patients (test set) from the Thrombolysis and Angioplasty in Myocardial Infarction (TAMI-5) Trial. Using stepwise logistic regression we were able to accurately predict the left anterior descending (LAD), right, or left circumflex (LC) coronary artery as the infarct-related artery using 2 variables: (1) the summation of the ST-segment elevation in leads V1 to V4; and (2) the summation of the T-wave negativity in leads I, aVL, and V5. In the development set, these 2 variables demonstrated respective sensitivity and specificity of 98% and 90% for LAD lesions, 82% and 85% for right narrowings, and 82% and 84% for LC narrowings. In the test set, the sensitivity and specificity were 97% and 95% for LAD lesions, 85% and 86% for right lesions, and 73% and 60% for LC coronary artery lesions. Information easily obtained on the ECG can accurately predict the likelihood of the LAD, right, or LC artery as the infarct-related artery. This may be useful in the decision to administer thrombolytic treatment.