Background and Objectives Cytokine-induced killer (CIK) cells have shown anti-leukemic activity and little graft-versus-host disease (GVHD) in several animal models. The safety of these cells in autologous settings has been shown. We performed a phase I study of allogeneic (donor’s) CIK cells in patients relapsing after allogeneic haematopoietic stem cell transplantation (HSCT). Design and Methods Eleven patients with acute myelogenous leukemia (n= 4), Hodgkin’s disease (n= 3), chronic myelomonocytic leukemia,(n= 1), pre-B acute lymphoblastic leukemia (n= 1) and myelodysplasia (n= 2), all of whom had relapsed after sibling (n= 6) or matched unrelated donor (n= 5) HSCT, entered this study. Results Before CIK administration, six patients had received other salvage treatments including chemotherapy (n= 5), radiotherapy (n= 1) and unmanipulated donor lymphocytes (n= 6) without any significant tumor response. The median number of CIK infusions was two (range 1–7) and the median number of total CIK cells was 12.4× 10 6/kg (range 7.2–87.4). The infusions were well tolerated and no acute or late infusion-related reactions were recorded. Acute GVHD (grade I and II) was observed in four patients, 30 days after the last CIK infusion, and progressed into extensive chronic GVHD in two cases. Disease progression and death occurred in six patients. One patient had stable disease, one had hematologic improvement and three achieved complete responses. Interpretation and Conclusions This study shows that the production of allogeneic CIK cells is feasible under clinical-grade conditions, well tolerated and may contribute to clinical responses.