Human leucocyte antigen (HLA)-Cw-reactive cytotoxic T lymphocytes (CTL) were generated from cord blood (CB) lymphocytes of two cases used for cord blood stem cell transplantation (CBSCT). In both cases, the CTL were cytotoxic against the patient's leukaemic cells, as well as the patient's Epstein-Barr virus (EBV)-lymphoblastoid cell line (EBV-LCL) and phytohaemagglutinin blasts, and the cytotoxicity was blocked by anti-HLA-class I monoclonal antibodies. In the first case, the CTL recognized Cw 3 (Cw 9 and Cw 10)-positive EBV-LCL, while in the second case, the CTL recognized Cw1 and/or Cw7. These cases suggest that CB T cells may be competent enough for generating CTL to induce a graft-versus-leukaemia effect and/or graft-versus-host disease in patients with CBSCT and that the mismatching of Cw antigens between patient and CB may be related to the outcome of CBSCT.