In patients with acute myocardial infarction (AMI), early ST segment elevation resolution on ECG predicts myocardial reperfusion and LV recovery. Intracoronary ECG is more sensitive than surface ECG to detect regional ischemia. In patients undergoing primary percutaneous coronary intervention (PCI), we investigated if failed myocardial reperfusion, despite successful infarct vessel recanalization, could be rapidly and easily identified by intracoronary ST segment monitoring from guidewire recording. We recorded intracoronary and standard ECG during primary coronary stenting (PCI) in 50 patients with AMI (59 +/- 11 years; anterior AMI in 66%). All patients had a successful PCI and underwent 2D echocardiography soon after PCI and 6 months later. Following PCI, intracoronary ST resolution >/= 50% from baseline was documented in 39 patients (78%; group A; from 11 +/- 8 to 1 +/- 2 mm) but not in 11 (22%; group B; from 11 +/- 8 to 8 +/- 5 mm). Group A had slightly shorter ischemic time (202 +/- 94 vs. 238 +/- 112 min in B; P = 0.2) and smaller peak CK values (2,752 +/- 2,038 vs. 4,802 +/- 3,671 U/L in B; P = 0.02). After PCI, ST resolution was found on standard ECG in 34 (87%) group A and in 3 (27%) group B patients. At 6-month follow-up, left ventricular ejection fraction was greater in group A (47% +/- 8% vs. 39% +/- 8% in B; P < 0.001) with improved wall motion score index (from 2.2 +/- 0.3 to 1.7 +/- 0.3 in A; from 2.3 +/- 0.4 to 2.1 +/- 0.4 in B; P < 0.001). There were no significant differences between intracoronary and standard ECG for sensitivity (92% vs. 86%) and specificity (62% vs. 57%) to predict improved infarct zone recovery after 6 months. ST elevation resolution on intracoronary recording during PCI predicts infarct zone recovery. Monitoring ST segment evolution by intracoronary ECG allows prompt and inexpensive identification in the catheterization laboratory of those patients without myocardial reperfusion, who may require adjunctive therapeutic interventions after successful infarct vessel recanalization.