This study was designed to find the effects of age on circulating endothelial progenitor cells (EPCs) and their mobilization in infants and young children following surgical correction of congenital heart defects. In 60 consecutive infants and young children (1month to 3years old) undergoing repair of atrial/ventricular septal defect, the numbers of EPCs and plasma levels of IL-6, -8, -10, TNF-alpha, VEGF and G-CSF were determined preoperatively, at the end of cardiopulmonary bypass (CPB), as well as 6, 12, 24, 48, 72 and 96h following surgery. Preoperative EPCs were reduced with increased age, similar to changes in plasma VEGF and G-CSF levels. Rapid mobilizations of EPCs and plasma VEGF, G-CSF were induced by cardiac surgery with CPB in all infants and young children, and the increased volumes of EPCs, VEGF and G-CSF decreased with age decreasing. The increased volumes of IL-6, -8, -10 and TNF-alpha were similar in different age groups. However, mobilization of EPCs, plasma VEGF and G-CSF were limited in infants <6months old, which did not correlate with change in inflammatory IL activation. Preoperative EPCs and plasma levels of VEGF and G-CSF were reduced with increasing age in infants and young children. Although a significant increase in EPCs and release of cytochemokines were observed in infants undergoing CPB, the mobilization of EPCs of the infants <6months old are limited.