The recently published papers about donor cell-derived leukemia after umbilical cord blood transplantation and related data were reviewed, while the mechanism of leukemia and related problems of umbilical blood conservation were discussed. In view of the possibility of prenatal origin of leukemia, it is necessary to more systematically check the leukemia relapse cases after umbilical cord blood transplantation for exactly identifying the frequency of donor cell derived leukemia and to determine whether the transfer of pre-leukemic clones is indeed present.