Electrocardiographic differentiation between left ventricular hypertrophy (LVH) and myocardial infarction (MI) is often difficult because both diagnoses are based primarily on QRS changes on the electrocardiogram (ECG). The specific goal of this study was the development of ECG criteria that could be used with the complete Selvester QRS scoring system for MI size in patients with LVH. A study population of 127 patients had significant aortic valve disease verified by cardiac catheterization. Inclusion in the study required no significant coronary artery disease, no focal contraction abnormality on the left ventriculogram, and no documented MI. Quantitative criteria for LVH developed by Bonner (IBM) and also those developed by the Cornell group were used to determine the ECG evidence for LVH in each patient. One or both sets of criteria were met in 110 (87%) of the 127 patients. This group was compared to a previously evaluated control population of 500 normal subjects. The complete 54-criteria, 32-point QRS MI size scoring system was applied to the 12-lead ECG of both groups. The score was 98% specific in the normal controls and 73% specific in the LVH group using a score of > 3 points as diagnostic for MI. Of the 54 individual QRS criteria, 16 failed to achieve 95% specificity in the LVH population: 13 were for anterior (and apical), 2 for inferior, and 1 for posterior locations. Of these 16, minor modifications to 11 were sufficient to achieve the 95% specificity standard. The resultant 49-criteria, 31-point QRS score for estimating MI size in patients with LVH was 95% specific using a level of > 3 points as diagnostic for MI. The sensitivity of this score and its ability to predict MI size in the presence of LVH must now be tested in prospective studies of patients with documented LVH, with and without MI. © 1992.