Umbilical cord blood transplantation(CBT) from HLA-mismatched unrelated donors has been increasingly performed. One of the advantages of unrelated CBT is a low risk of severe acute graft-versus-host disease(aGVHD). The degree of HLA disparities is not strongly associated with the occurrence of severe aGVHD. The disadvantages of CBT include a delayed time of hematopoietic recovery and a high rate of graft failure, which are significantly associated with the lower number of infused cord blood cells. Despite a relatively high rate of early transplant-related mortality, cord blood cells from HLA-mismatched unrelated donors should be considered as an alternative hematopoietic stem source for both children and adults who have no suitable related and unrelated bone marrow donors. Further clinical and laboratory studies are needed to improve the outcome of CBT.