We investigated early temporal changes in end-diastolic wall thickness (EDWT) of the infarcted myocardium in 46 patients with reperfused anterior acute myocardial infarction in relation to myocardial viability. Two-dimensional echocardiography was performed on days 1 and 2 of acute myocardial infarction, and the EDWT of the anterior segment was measured in the short-axis view. Patients were divided into three groups on the basis of day 1 to day 2 ratio of EDWT: the ratio ≤0.85 as group A (n = 13), >0.85 but <1.15 as group B (n = 23), and >1.15 as group C (n = 10). Left ventricular functional improvement was significantly better in group B than in groups A and C. Substantial size of “no reflow” phenomenon was observed only in groups A (n = 9, 69%) and C (n = 6, 60%). The frequency of transient ST re-elevation after reperfusion was the highest in group C (70%), and left ventricular expansion was observed at day 2 only in group A. We conclude that changes in the EDWT of the infarct segment early after reperfusion, either decreases or increases, are related to irreversibly damaged myocardium. A decrease in EDWT and concomitant ventricular expansion may be related to impaired myocardial perfusion. An increase in EDWT after reperfusion may be caused by accelerated myocardial and microvascular damage after reperfusion.