Hematopoietic stem cell transplantation (HSCT) is becoming an effective therapeutic modality for a variety of diseases. Mesenchymal stem cells (MSCs) can be used to enhance hematopoietic engraftment, accelerate lymphocyte recovery, reduce the risk of graft failure, prevent and treat graft-versus-host disease, and repair tissue damage in patients receiving HSCT. Till now, most MSCs for human clinical application have been derived from bone marrow. However, acquiring bone-marrow-derived MSCs involves an invasive procedure. Umbilical cord is rich with MSCs. Compared to bone-marrow-derived MSCs, umbilical cord-derived MSCs (UCMSCs) are easier to obtain without harm to the donor and can proliferate faster. No severe adverse effects were noted in our previous clinical application of UCMSCs in HSCT. Accordingly, application of UCMSCs in humans appears to be feasible and safe. Further studies are warranted.