The clinical significance of additional ST segment elevation immediately after reperfusion in acute myocardial infarction (AMI) is still unclear. The influence of additional ST elevation on myocardial damage was examined in 62 patients with first anterior AMI. All patients had coronary reflow (thrombolysis in myocardial infarction: TIMI grade III) within 6 hours after the onset of infarction and no subsequent reocclusion for at least 14 days. The patients were classified into two groups: group ST (+), 26 patients with additional ST elevation (> or = 5 mm increase in sigma ST in lead V1-V6) immediately after reperfusion, and group ST(-), 36 patients without additional ST elevation. Myocardial damage was estimated by the number of abnormal Q wave before and 1 hour after reperfusion (nQpre and nQpost. There were no significant differences in the elapsed time, ECG indexes before reperfusion and regional wall motion (RWM: SD/chord; SD = standard deviation) in the acute phase and 14 days after the onset between the two groups. Moreover, within each group the patients were classified into two groups on the basis of RWM 14 days after the onset: group A = RWM < -2.5 in group ST (+) (n = 17); group B = RWM > or = -2.5 in group ST(+) (n = 9); group C = RWM < -2.5 in group ST (-) (n = 14); group D = RWM > or = -2.5 in group ST(-) (n = 22). There were no significant differences in the elapsed time between the four groups. nQpre was highest in group C and was lowest in group D (group A: 2.1, group B: 1.9, group C: 4.1, group D: 1.4), and nQpost was highest in group C and group A, and was lowest in group D (group A: 4.1, group B: 2.7, group C: 4.4, group D: 2.0). There was a significant increase in nQ after reperfusion (nQpost-nQpre) in group A and group B, which was greater in group A than in group B. The increase in sigma ST after reperfusion was greater in group A than in group B. There was no significant increase in nQ after reperfusion in group C and in group D, and both nQpre and nQpost were highest in group C, and lowest in group D. Infarct size was larger in group A and in group C than group B and group D, and there was no significant improvement in RWM in group A and in group C. In conclusion, additional ST elevation immediately after reperfusion seems to occur in patients with not so severe myocardial damage before reperfusion, and may reflect reperfusion injury. Additional ST elevation is not observed in patients with preceding extensive myocardial damage which may be irreversible or not still advanced at the time of reperfusion.