Background The implication of the shape of ST elevation in the acute phase of myocardial infarction (MI) remains unclear. Methods and Results We examined the relation between the shape of ST elevation and infarct size in 77 patients who had a first acute anterior MI with successful reperfusion within 6 hours from symptom onset. A 12-lead electrocardiogram was recorded immediately before reperfusion confirmed by coronary angiography. The shape of ST elevation in lead V₃ was classified into 3 types: concave type (n = 24), straight type (n = 41), and convex type (n = 12). For concave type, straight type, and convex type, a median value of peak creatine kinase was 2287, 4371, and 5322 mU/mL, and left ventricular ejection fraction measured by left ventriculography at discharge (14 days after MI) was 58%, 48%, and 41% (P < .05; concave type versus the other 2 types), respectively. A multivariate logistic regression model demonstrated that the concave type of ST elevation was a strong predicting factor for preserved left ventricular function (left ventricular ejection fraction ≥50% at discharge; odds ratio 6.2, 95% confidence interval 1.6 to 20.8, P = .019). Conclusions In patients with reperfused acute anterior MI, left ventricular function was excellent in patients with concave type, intermediate in those with straight type, and relatively poor in those with convex type ST elevation at discharge. This simple classification is useful for predicting left ventricular function at discharge. (Am Heart J 1999;137:522-7.)