Allogeneic bone marrow transplantation in patients with leukaemia is an aggressive therapeutic procedure which implies high early mortality. Current opinion trends attribute the greater part of the procedure toxicity to the preparative regimen. The results of a multivariate analysis on data of 174 leukaemia patients conditioned with total body irradiation (TBI), 10-12 Gy, single dose or fractionated, and lung shielding at 8 Gy, plus chemotherapy: cyclophosphamide 120 mg/kg, before or after TBI, are presented. The variables statistically related with early mortality are age, Karnofsky index (KI) and acute graft-versus-host disease (GvHD). No variable depending on radiotherapy reached the significance level. The relative risk of early mortality for patients older than 26 years, in bad general condition (KI < 90%), or developing acute severe (grades II-IV)GvHD, is 3.99, 5.68, and 6.71, respectively. We conclude that in the range of TBI schedules analysed, radiation therapy is not an important factor in early death, but acute severe GvHD, or recipient's bad general condition are factors to be improved by bone marrow transplantation teams if they want to improve the therapeutic index of the procedure.