Cytokine-induced killer (CIK) cells are CD3(+)CD56(+) non-major histocompatibility complex (MHC)-restricted immune effector cells. The present report demonstrates that it was possible to expand CIK cells obtained at diagnosis from patients with acute leukaemia. The percentage of CD3(+)CD56(+) CIK cells generated following culture ranged between 7.6% and 65% (median of 35.3%) and these cells were able to kill the human natural killer target K562 cells. Although the same effector cells were able to lyse autologous acute myeloid leukaemia (AML) target cells, they were not able to lyse autologous acute lymphoblastic leukaemia target cells. Pre-absorption of the CIK effector cells by K562 cells did not completely abrogate the cytotoxicity of CIK cells against autologous blasts in 9 out of 12 samples tested. Moreover, it was observed that the cytotoxicity generated by the CIK effector cells against allogeneic leukaemic blasts was similar to that against autologous blasts. The present study suggests the potential application of CIK cells in the immunotherapy of AML, either in minimal disease state, as donor lymphocyte infusion in relapse post allogeneic transplant, or in cases of chemotherapy refractory leukaemia.