Peripheral blood of adult species contains endothelial progenitor cells (EPCs) that participate in neovascularization, consistent with postnatal vasculogenesis. EPCs can be isolated not only from peripheral blood but also from bone marrow and human umbilical cord blood. In vitro culture-expanded EPCs participate in endothelial network formation (capillary formation) in vitro, and transplanted EPCs have been incorporated into sites of active neovascularization. For example, transplanted human EPCs formed capillaries among preserved skeletal myocytes in the ischemic hindlimb of athymic nude rats in vivo. Furthermore, transplantation of EPCs functionally augmented neovascularization in response to hindlimb ischemia. Thus, transplantation of EPCs may become a useful strategy to modulate postnatal neovascularization.