Electrocardiography is an excellent tool for decision making in patients with ST elevation myocardial infarction (STEMI). However, little is known on the correlation between its dynamic changes during primary percutaneous coronary intervention (PCI) and the anatomic information provided by cardiovascular magnetic resonance. The study aimed to assess the predictive value of dynamic ST-segment changes before and after PCI on myocardial area at risk (AAR), infarct size, and left ventricular function in patients with STEMI. Eighty-five consecutive patients with a first STEMI were included. An electrocardiogram was recorded before and after PCI at 1, 24, 48, 72, and 120 hours. Sum of ST elevation (sumSTE), the number of STE, and STE resolution (resSTE) were determined. Complete resSTE was defined as ≥70% resolution, and patients were classified into 3 groups: group 1 (resSTE 1 hour after PCI) n = 39; group 2 (resSTE 120 hour after PCI) n = 27; and group 3, without resSTE (n = 19). Cardiovascular magnetic resonance was performed during hospitalization and at 6 months. Left ventricular volumes, ejection fraction, AAR, infarct size, myocardial salvage index, and microvascular obstruction were determined. Before PCI, the number of STE and sumSTE were best associated with AAR (p <0.001). After PCI, lack of resSTE (group 3) was associated with larger infarct size, MVO, and lower myocardial salvage index. However, sumSTE at 120 hours after PCI best discriminated patients with larger infarct size, ventricular volumes, and lower ejection fraction during hospitalization and at follow-up. In conclusion, admission sumSTE best correlates with AAR, whereas sumSTE at 120 hours rather than early resSTE best correlates with infarct size and left ventricular volumes during hospitalization and at 6 months.