Vasculogenesis, the process by which endothelial cell precursors are recruited and organized to form a vasculature, has traditionally been thought to play a role only in embryonic development. However, several studies have now been published suggesting that vasculogenesis may have a role in the formation of new vascular networks during postnatal life. Recent studies suggest the existence of circulating endothelial precursor cells that arise from outside the place of vascularization. Using a mouse bone marrow (BM) transplantation model that takes advantage of MHC haplotype differences between donor and recipient mice, we examined the contribution of donor BM-derived cells to neovascularization in recipient nude mice with developing Ewing's sarcoma tumors. We found that the donor BM cells gave rise to endothelial cells in vitro and colocalized with neovessels in Ewing's sarcomas in vivo. We also found that donor BM-derived cells were involved in the formation of the tumor vasculature. Our findings indicate that not only angiogenesis but also vasculogenesis was involved in the development of Ewing's sarcoma in our mouse model.