Rapid resolution of ST-segment elevation is a reperfusion-associated electrocardiographic change in acute myocardial infarction. However, some patients have additional ST-segment elevation immediately after reperfusion before such resolution. The clinical significance and the effect on myocardial salvage of this electrocardiographic change are unknown. To examine this electrocardiographic feature and determine its clinical basis for occurrence and influence on left ventricular function, 58 consecutive patients with a first anterior wall acute myocardial infarction who had intracoronary thrombolysis or coronary angioplasty, or both, were assessed. With the use of frequent electrocardiographic procedures during reperfusion therapy, patients were divided in 2 groups: those with additional ST-segment elevation (n = 35; group A, > or = 0.5 mV increase in summed ST-segment elevation in lead V1-V6 within 15 minutes after reperfusion), and those without this phenomenon (n = 23; group B). Baseline characteristics, creatine kinase kinetics and left ventricular function were compared between both groups. Before reperfusion, group A had a greater summed ST-segment elevation (2.44 +/- 1.07 vs 1.57 +/- 0.98 mV; p = 0.003) and poorer collaterals (p = 0.001) than did group B. Peak creatine kinase was significantly higher in group A than in B (6,550 +/- 3,477 vs 4,310 +/- 1,880 IU/liter; p = 0.003). Group A had less improvement in ejection fraction (-4.2 +/- 9.9% vs 1.7 +/- 9.6%; p = 0.04) and regional wall motion (0.28 +/- 0.74 vs 0.76 +/- 0.79 SD/chord; p = 0.03) than did group B. It is thought that additional ST-segment elevation immediately after reperfusion occurred in myocardium with severe ischemic damage before reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)