Today, bone marrow transplantation (BMT) is an established therapy. This statement is best verified by the number of BMTs performed. Between January 1990 and December 1992, 172 European teams in 26 countries carried out a total of 14,334 transplants. There were 6642 allogeneic transplants: 5513 BMT from an HLA-identical sibling donor, 370 from a non-identical family member, 88 from an identical twin donor and 671 from an unrelated volunteer donor. There were 7692 autologous transplants: 6577 autologous bone marrow, 777 peripheral-blood stem-cell and 338 combined bone-marrow and peripheral-blood stem-cell transplants. Indications were: leukaemias in 52% (7479), lymphoproliferative disorders in 29% (4125), solid tumours in 11% (1540), aplastic anaemia and thalassaemia in 3% (487) and inborn errors an miscellaneous disorders in the remaining 5% (703). The results of these transplants are not yet known. From previous analyses it can be expected that more than 50% of patients will be alive and well 10 years after BMT. The main factors influencing outcome are known; they depend on type, sub-type, stage of disease at time of transplant, the time from diagnosis to transplant and the conditioning regimen for all transplants. For allogeneic BMT, donor source, donor and recipient age, sex, donor/recipient sex combination, donor and recipient viral status, graft-versus-host disease prevention method and region are additional factors. Knowledge of these factors enables us today to estimate the potential risk and adjust the therapy for an individual patient.