Based on previous studies, donor NK cells will be eliminated in most cases by the resurgent cellular immunity of the haploidentical recipient after the effects of the transient immunosuppression caused by the conditioning regimen have ceased (typically within 1 month of infusion). Conceivably, however, NK cell engraftment may persist for a longer period with the possible risk of prolonged pancytopenia owing to NK cell killing on normal hematopoietic cells. Because of this possibility, we will plan for HSCT rescue in patients receiving redirected NK cell infusions. Indeed, for most of the patients eligible for this study, HSCT would be the treatment intervention regardless of whether they receive NK cell infusions or not. Because of these considerations, the potential benefits of NK cell therapy should outweigh its risks for those patients eligible for this study, i.e., patients with persistent leukemia for whom no other proven effective treatment is available.