Endothelial progenitor cells (EPCs) are released from the bone marrow during cardiac ischemic events, potentially influencing vascular and myocardial repair. We assessed the clinical and angiographic correlates of EPC mobilization at the time of primary percutaneous coronary intervention in 78 patients with ST elevation myocardial infarction and the impact of both baseline and follow-up EPC levels on left ventricular (LV) remodeling. Blood samples were drawn from the aorta and the culprit coronary artery for cytofluorimetric EPC detection (CD34+CD45dimKDR+ cells, in percentage of cytofluorimetric counts). Area at risk was assessed by Bypass Angioplasty Revascularization Investigation myocardial jeopardy index, thrombotic burden as thrombus score and microvascular obstruction (MVO) as a combination of ST segment resolution and myocardial blush grade. Echocardiographic evaluation of LV remodeling was performed at 1-year follow-up in 54 patients, whereas peripheral EPC levels were reassessed in 40 patients. EPC levels during primary percutaneous coronary intervention were significantly higher in intracoronary than in aortic blood (0.043% vs 0.0006%, p <0.001). Both intracoronary and aortic EPC were related to area at risk extent, to intracoronary thrombus score (p <0.001), and inversely to MVO (p = 0.001). Peripheral EPC levels at 1-year follow-up were lower in patients with LV remodeling than in those without (0.001% [0.001 to 0.002] vs 0.003% [0.002 to 0.010]; p = 0.01) and independently predicted absence of remodeling at multivariate analysis. In conclusion, a rapid intracoronary EPC recruitment takes place in the early phases of ST elevation myocardial infarction, possibly reflecting an attempted reparative response. The extent of this mobilization seems to be correlated to the area at risk and to the amount of MVO. Persistently low levels of EPC are associated to LV remodeling.