Peripheral blood of adult species contains endothelial progenitor cells (EPCs) that participate in neovascularization, consistent with postnatal vasculogenesis. Abundant EPCs can be isolated from a relatively small volume of human umbilical cord blood, and that culture-expanded EPCs participate in endothelial network formation in vitro. Transplanted EPCs incorporated into sites of active neovascularization and formed capillaries among preserved skeletal myocytes in the ischemic hindlimb of athymic nude rats in vivo. Furthermore, transplantation of EPC quantitatively and effectively augmented neovascularization in response to hindlimb ischemia. Thus, human umbilical cord blood seems to be a novel source for EPCs, and the transplantation of cord blood-derived EPCs may become a useful strategy to modulate postnatal neovascularization.