AIMS: Assessment of microvascular function in patients with ST-elevation acute myocardial infarction (STEMI) may be useful to determine treatment strategy. The possible role of pressure-bounded coronary flow reserve (pb-CFR) in this setting has not been determined. METHODS AND RESULTS: Thermodilution-pressure-wire assessment of the infarct-related artery was performed in 148 STEMI patients before stenting and/or at completion of primary percutaneous coronary intervention (PPCI). The extent of the myocardial injury was assessed with cardiovascular magnetic resonance imaging at 48-hours and 6-months after STEMI. Post-PPCI pb-CFR was impaired (<2) and normal (>2) in 69.9% and 9.0% of the cases respectively. In the remaining 21.1% of the patients, pb-CFR was "indeterminate". In this cohort, pb-CFR correlated poorly with thermodilution-derived coronary flow reserve (k=0.03, p=0.39). The index of microcirculatory resistance (IMR) was significantly different across the pb-CFR subgroups. Similarly, significant differences were observed in microvascular obstruction (MVO), myocardium area-at-risk and 48-hours infarct-size (IS). A trend towards lower 6-month IS was observed in patients with high (>2) post-PPCI pb-CFR. Nevertheless, pb-CFR was inferior to IMR in predicting MVO and the extent of IS. CONCLUSIONS: Pb-CFR can identify microvascular dysfunction in patients after STEMI and provided superior diagnostic performance compared to thermodilution-derived CFR in predicting MVO. However, IMR was superior to both pb-CFR and thermodilution-derived CFR and consequently, IMR was the most accurate in predicting all of the studied CMR endpoints of myocardial injury after PPCI.