This study correlated the location and size of posterolateral myocardial infarcts (Mls) measured anatomically with that estimated by quantitative criteria derived from the standard 12-lead ECG. Twenty patients were studied who had autopsy-proved, single, posterolateral Mls and no confounding factors of ventricular hypertrophy or bundle branch block in their ECG. Left ventricular anatomic Ml size ranged from 1 to 46%. No patient had a ≥ 0.04-second Q wave in any electrocardiographic lead and only 55% had a 0.03-second Q wave. A 29-point, simplified QRS scoring system consisting of 37 weighted criteria was applied to the ECG. Points were scored by the ECG in 85% of the patients (range 1 to 8 points). Ml was indicated by a wide variety of QRS criteria; 19 of the 37 criteria from 8 different electrocardiographic leads were met. The correlation coefficient between MI size measured anatomically and that estimated by the QRS score was 0.72. Each point represented approximately 4% Ml of the left ventricular wall. © 1983.