The sum of ST-segment elevation (ΣST on V_2-4) was measured to evaluate ST-segment re-elevation during early convalescence in 57 patients with acute myocardial infarction. Following rapid ST-segment elevation resolution during the first 12 h, ΣST again increased in many patients without signs of reinfarction or pericarditis, reaching a maximum approximately 5 days after onset. The magnitude of this re-elevation (Δ ΣST) was less than 0.3 mV in 30 patients (group A), and 0.3 mV or more in another 27 (group B). Based upon left ventriculography, the global ejection fraction in group B decreased significantly from 51±10% at the acute phase to 46±10% at the chronic phase. No such decreases were seen for group A. Regional ejection fraction in the infarcted portion improved significantly from 28±13% at the acute phase to 35±14% at the chronic phase in group A, but did not improve in group B. In addition, the non-infarcted portion in group B showed a significantly reduced regional ejection fraction. These results suggest that myocardial expansion of the infarcted portion may contribute to ST-segment re-elevation, an ominous sign of left ventricular dysfunction soon after acute myocardial infarction.