Decreased graft-versus-host disease (GVHD), ease of accessibility, and sustained engraftment encourage the use of umbilical cord blood (UCB) as an alternative source to bone marrow for immune reconstitution in children with leukemia. Natural killer (NK) cells rapidly expand after stem cell transplantation and are important for regulating GVHD and providing graft-versus-leukemia (GVL) effects. This review highlights the phenotypic and functional differences between UCB NK cells and adult peripheral blood (APB) NK cells, and discusses the possible therapeutic benefit of using UCB NK cells for adoptive immunotherapy in leukemia. Alloreactive NK cells show potent cytotoxic activities against human leukocyte antigen (HLA)-nonidentical leukemic cells and reduce leukemia relapses. The higher numbers of NK progenitors in UCB makes it a convenient source for ex vivo expansion of UCB NK cells for posttransplant treatment. UCB NK cells readily respond to interleukin-15, which may greatly enhance their antitumor effect. Activation and expansion protocols for UCB NK cells are currently being developed.