Aim: We investigated the relationship between coronary collateral formation and circulating endothelial progenitor cells (EPC) in patients undergoing coronary angiography. Methods and results: Circulating CD133 +/34 + and CD34 +/KDR + EPCs were determined in 68 patients (normal coronary vessels in 24 patients and coronary artery disease (CAD) in 44 patients) (age: 58.7±10.1, 64.7% male). Circulating EPCs were higher among patients with normal coronary vessels compared to patients with CAD for CD133 +/34 + (p<0.05) and CD34 +/KDR + cells (p<0.05). The number of EPCs were significantly greater in patients with good coronary collateral formation (p<0.05). EPC count was independent predictor for coronary collateral formation after adjustment for other cardiovascular risk factors and extent of CAD (p=0.037). Conclusion: In patients with severe coronary stenosis, those with increased circulating EPCs had better collateral formation compared to those with lower EPC counts. Our findings implicate that in addition to presence of critical stenosis, intact response of bone marrow is necessary for collateral formation in CAD. © 2011 Elsevier Ireland Ltd.