1957 allogeneic HLA-identical sibling-donor bone-marrow transplants done in 52 European centres between 1979 and 1986 and reported to the European bone-marrow transplant leukaemia registry were analysed. The most important factor influencing leukaemia-free survival, transplant-related mortality, and relapse incidence was the stage of the disease at the time of the transplant. This dominant role of the stage of the disease in all three diagnostic categories--acute myeloblastic leukaemia, acute lymphoblastic leukaemia, and chronic myeloid leukaemia--for all three end-points clearly indicates that resistance of the leukaemia cell to the procedure is more important than bulk of the disease. Additional prognostic factors for leukaemia-free survival and transplant-related mortality were age of the patient, cyclosporin for preventing graft-versus-host disease, and the donor-recipient sex combination. The risk of relapse was highest in patients with acute lymphoblastic leukaemia. In a multivariate analysis leukaemia-free survival was similar for all three major diagnostic categories and has not changed since the introduction of the European registry in 1979. These results show that the biological differences between the three main diagnostic categories of leukaemia are not as great as had been assumed and that the traditional approaches to improving results in recent years have failed.