Five patients with aggressive acute leukemias who had relapsed after cord blood transplantation were treated with cord blood derived cytokine-induced killer (CIK) cells. These were obtained by ex vivo expansion, using as starting material the washouts of the cord blood units, left over at the end of the transplant. We did not observe any acute or delayed adverse event, and observed 1 partial response in 1 patient concomitantly with the development of acute grade III graft-versus-host disease (GVHD). These observations show the relatively low toxicity of cord blood-derived CIK cells and, more importantly, the feasibility of this immunotherapy program for patients who could not otherwise benefit from donor lymphocyte infusions.