Background: Endothelial progenitor cells (EPCs) are bone marrow-derived cells that are augmented in response to ischemia and incorporated into neovascularization sites. We sought to determine whether circulating EPCs are related to collateral formation following non-ST segment elevation myocardial infarction (NSTEMI). Methods: Twenty patients who underwent percutaneous coronary intervention (PCI) within a week of NSTEMI were divided into two groups: patients without collaterals (coll–, n = 10) and patients with Rentrop grade 3–4 collaterals (coll+, n = 10). Blood samples were drawn before PCI and 24 ± 2 h after PCI. EPC colonies were grown from peripheral blood mononuclear cells, characterized, and counted. Using flow cytometry the percentage of cells coexpressing vascular endothelial growth factor receptor-2 and CD133 was determined. Results: The coll+ group had higher degree of culprit vessel stenosis and lower initial thrombolysis in myocardial infarction flow grade. The relative number of EPCs before PCI was significantly higher in the coll+ group than in the coll– group (1.49 ± 0.9% vs. 0.77 ± 0.4%, p = 0.045). There were no significant intergroup differences in the number of EPC colony-forming cells. The number of EPC colonies increased in the coll– group after PCI (9.5 ± 4.8 to 14.0 ± 5.9/10<sup>6</sup> cells, p = 0.01). Conclusions: This study supports an association between circulating EPC levels and collateral formation in patients with an NSTEMI.