Ex vivo depletion of lymphocytes has been used to decrease the incidence of acute graft-versus-host disease following the allogeneic bone marrow transplantation. Many depletion techniques also remove natural killer (NK) cells that may mediate a beneficial graft-versus-leukemia effect. In this paper we review our studies on ex vivo expansion and activation of NK cells for use in the therapy of chronic myelogenous leukemia.