One-hundred and five patients undergoing allogeneic bone marrow transplantation (BMT) for acute myeloid leukaemia (AML) (n = 61) and chronic myeloid leukaemia (n = 44) were analysed for risk factors associated with relapse. All patients received marrow from an HLA identical sibling after preparation with cyclophosphamide 120 mg/kg and total body irradiation (TBI) 330 cGy on each of the three days prior to transplantation. There was a difference of +/- 18% between the nominal total dose of 990 cGy and the actual dose received as indicated by dosimetric recordings. While interstitial pneumonitis had minimal impact on survival (4%) there was a considerable difference in the incidence of relapses. The incidence of relapse was 55% versus 11% in patients receiving less or more than 990 cGy respectively and this had a major impact on survival (38% v. 74% at 7 years) since transplant-related mortality was comparable in the two groups. A multivariate Cox analysis indicated that a lower TBI dose (less than 990 cGy) was the most significant factor associated with relapse and the second most important factor associated with recurrence of leukaemia was the absence of chronic graft-versus-host-disease (cGvHD). Actuarial relapse incidence was 62%. 28% and 18% for patients with no, limited or extensive chronic GvHD respectively. However, chronic GVHD had no significant impact on survival. Combined stratification for TBI dose and cGvHD showed that the dose effect of TBI on relapse was evident both in patients with and without cGvHD. Chronic GvHD influenced the risk of relapse only in patients receiving less than 990 cGy. These results suggest that a higher dose of TBI, within this schedule, produced long-term disease-free survival in the majority of AMLs and CMLs. Minor radiobiological side effects were experienced but a small reduction of the dose may significantly increase the risk of relapse.