There is growing interest in using human umbilical cord blood (CB) for allogeneic bone marrow transplantation (BMT), particularly in children. Thus, CB has been identified as a rich source of hematopoietic progenitors of the erythroid, myeloid, and B-cell lineages. Whether CB blood cells engrafting in the BM space also comprise T-cell progenitors capable of trafficking to the thymus and reconstituting a functional thymopoiesis in young recipients is presently unknown. Here, we show that CB progenitors, engrafted in the BM of immunodeficient mice, sustain human thymopoiesis by generating circulating T-cell progenitors capable of homing to and developing within a human thymic graft. Surprisingly, development of CB stem cells in this in vivo model extended to elements of the endothelial cell lineage, which contributed to the revascularization of transplants and wound healing. These results demonstrate that human CB stem cell transplantation can reconstitute thymic-dependent T-cell lymphopoiesis and show a novel role of CB-derived hematopoietic stem cells in angiogenesis.