Previously developed formulas for predicting the final QRS estimated sizes of acute myocardial infarcts from the initial ST segment deviation are tested. The population contains patients with either anterior or inferior infarcts from two hospitals in Copenhagen, Denmark. The formula for anterior location that considers only the number of ECG leads with ST elevation achieved a high correlation (r = 0.70). However, the formula for inferior location that considers the quantity of ST elevation in inferior leads achieved a lower value (r = 0.52). Empiric modifications of this formula were constructed that considered either the quantity of ST elevation in (A) or the number of (B) involved non-inferior leads. Each modification achieved improvement in correlation with QRS estimated MI size in the original population (A: r = 0.63 and B: r = 0.65), and also in an independent test population (A: r = 0.57 and B: r = 0.62). These results suggest that the formula of Aldrich et al. for anterior location is valid for clinical application, but that further studies are required to determine if a comparably accurate method can be developed for inferior MI.