BACKGROUND:Circulating microRNAs (miRs) have emerged as potential diagnostic markers in patients with myocardial infarction. Previous studies, however, were based on limited patient numbers and could not assess the relation of miRs to myocardial damage. Moreover, the prognostic value of miRs in ST-elevation myocardial infarction (STEMI) is unknown. The aims of this study were (1) to assess the relation between miR-133a and myocardial damage assessed by cardiovascular magnetic resonance (CMR) imaging and (2) to evaluate the prognostic value of miR-133a in reperfused STEMI. METHODS:MicroRNA-133a concentrations were determined in 216 consecutive patients with STEMI undergoing primary angioplasty less than 12 hours after symptom onset. Patients were categorized into 2 groups defined by the median miR-133a value on admission. Cardiovascular magnetic resonance was performed for assessment of infarct size, myocardial salvage, and microvascular obstruction. The primary clinical end point was the occurrence of major adverse cardiovascular events defined as a composite of death, reinfarction, and new congestive heart failure within 6 months after infarction. RESULTS:All prognostic relevant CMR markers (infarct size, microvascular obstruction, myocardial salvage index) showed significant correlations with circulating miR-133a concentrations (P < .001 for all).The strongest predictors of miR-133a concentrations were the time from symptom onset to reperfusion and the amount of the salvaged area at risk. Major adverse cardiovascular events occurred significantly more often in the miR-133a ≥ median group (9% vs 20%, P = .025). However, miR-133a concentrations were unable to independently predict clinical events. CONCLUSIONS:Elevated levels of circulating miR-133a in patients with STEMI are associated with decreased myocardial salvage, larger infarcts, and more pronounced reperfusion injury. Consequently, miR-133a concentrations can provide prognostic information but do not add independent prognostic information to traditional and CMR markers of clinical prognosis in a high-risk STEMI population.