Umbilical cord blood (UCB) has become a frequent source of allogeneic hematopoietic stem cells for transplantation. Of theoretical concern is a potential decrease in the graft vs. leukemia (GvL) effect, given the lesser degree of graft vs. host disease (GvHD) with this donor source. We report a case of recurrent acute non-lymphoblastic leukemia (ANLL) following stem cell transplantation with unrelated mismatched UCB, which responded to the induction of GvHD. The response was documented both morphologically and by evaluation of chimeric engraftment by molecular DNA techniques. In addition, WT-1, a purported marker of minimal residual disease in acute leukemia, correlated with remission status in this patient. In summary, the GvL effect is seen with allogeneic UCB transplantation and has the potential to be induced along with GvHD.