A 31-year-old woman with Philadelphia (Ph) chromosome-positive chronic myeloid leukaemia (CML) underwent allogenic bone marrow transplantation during accelerated phase. Non-T-cell-depleted marrow from a male sibling mismatched at one Class 2 histocompatibility locus was infused after conditioning with total body irradiation and intravenous cyclophosphamide. Cyclosporin and methotrexate were given for prevention of graft-versus-host disease (GVHD). Prompt engraftment occurred with donor karyotype cells, followed by transient moderate acute GVHD. However, by day 60 after BMT, haematological relapse occurred with increasing splenomegaly, leucocytosis, increasing marrow fibrosis, and cytogenetic mosaicism, consisting of 47% donor metaphases with 53% Ph-positive host metaphases, some containing additional structural changes. Thirty days later further cytogenetic progression was evident. A slowly progressive fungal pneumonia concurrently present was treated with intravenous amphotericin and gradual reduction of cyclosporin. Subsequently, without further cytotoxic chemotherapy, pancytopenia and bone marrow hypoplasia developed, and on day 144 only donor karyotype marrow cells were seen. Chromosomes have remained of donor type on subsequent occasions, and the patient has a normal performance status 25 months after BMT. The patient's course illustrates that factors operating after allogeneic BMT contribute to longterm control of CML. The factors potentially responsible for this spontaneous remission, after early relapse, are discussed.