This report summarizes the results of marrow transplantation from HLA-identical siblings and syngeneic twins for treatment of acute myelogenous leukaemia, chronic myelogenous leukaemia, acute lymphoblastic and undifferentiated leukaemia from 1975 until December 1986. Three conditioning regimens and treatment of the marrow graft in vitro with absorbed antithymocyte globulin or the monoclonal antibody "Campath 1" for prophylaxis of graft-versus-host disease (GVHD) have been studied and analyzed retrospectively. The regimen of total body irradiation in large fractions of 4 Gy and of cyclosphosphamide (200 mg/kg) has achieved the most favorable results. Inactivation of T-cells by treatment of the marrow "in vitro" has decreased the severity of GVHD without improving survival. The antileukaemic effect of the graft may be important for control of the disease and may be improved by better immunosuppression of the recipient.