A relatively large number of patients with chronic myeloid leukaemia (CML) have now been treated in different centres by transplantation of bone marrow from HLA-identical sibling donors. The 4-year leukaemia-free survival is 45 to 55%. The major determining factors are disease status at diagnosis, patient age, the occurrence of graft-versus-host post-transplant and the use of T-cell depletion. Haematological or cytogenetic relapse is rare in patients transplanted with T-replete marrow in chronic phase but common in recipients of T-depleted donor marrow cells. This observation provides strong evidence for the importance of a graft-versus-leukaemia effect, which may be especially important after allogeneic bone marrow transplantation for CML. One major outstanding problem is how to determine the optimal timing of transplantation within the chronic phase.