Background Patients with acute coronary occlusion (ACO) may not only have ischemia‐related ST‐segment changes but also changes in the QRS complex. It has recently been shown in dogs that a greater ischemic QRS prolongation (IQP) during ACO is related to lower collateral flow. This suggests that greater IQP could indicate more severe ischemia and thereby more rapid infarct development. Therefore, the purpose was to evaluate the relationship between IQP and measures of myocardial injury in patients presenting with acute ST‐elevation myocardial infarction (STEMI).Methods Seventy‐seven patients with first‐time STEMI were retrospectively included from the recently published SOCCER trial. All patients underwent a cardiac magnetic resonance (CMR) examination 2–6 days after the acute event. Infarct size (IS), myocardium at risk (MaR), and myocardial salvage index (MSI) were assessed and related to IQP. IQP measures assessed were; computer‐generated QRS duration, QRS duration at maximum ST deviation, absolute IQP and relative IQP, all derived from a pre‐PCI, 12‐lead ECG.Results Median absolute IQP was 10 ms (range 0–115 ms). There were no statistically significant correlations between measures of IQP and any of the CMR measures of myocardial injury (absolute IQP vs IS, r = 0.03, p = 0.80; MaR, r = −0.01, p = 0.89; MSI, r = −0.05, p = 0.68).Conclusions Unlike previous experimental studies, the IQP was limited in patients presenting at the emergency room with first‐time STEMI and no correlation was found between IQP and CMR variables of myocardial injury in these patients. Therefore, IQP does not seem to be a suitable biomarker for triaging patients in this clinical context.