In patients with acute myocardial infarction (AMI), transient QT interval prolongation with a prominent negative T wave is frequently observed in cases of early spontaneous reperfusion and often indicates a good prognosis. Additionally, in nuclear cardiac imaging, technetium-99m/thallium-201 overlap on dual single-photon emission computed tomography (dual SPECT) in AMI patients indicates the presence of viable myocardium and early recanalization. To elucidate the clinical significance of this transient QT interval prolongation, 34 patients (64±8 years) admitted within 24 h of the onset of anterior AMI were enrolled and classified into 2 groups according to the presence (group A, n=24) or absence (group B, n=10) of scintigraphic overlap on simultaneous dual SPECT imaging. The maximal QTc interval was 0.59±0.06 s in group A and 0.52±0.06 s in group B (p<0.01). The peak creatine kinase was lower in group A (2650±2160 IU/L) than in group B (3490±2060 IU/L). The left ventricular ejection fraction (LVEF) at discharge was 62±11% in group A and 49±14% in group B (p<0.01). The scintigraphic overlap group had a smaller infarct and better LVEF, which suggests that the QT interval prolongation that appears transiently in the acute phase of AMI indicates scintigraphically the presence of salvaged myocardium. (Jpn Circ J 2001; 65: 7 -10)