Donor-derived leukaemia is exceptional after allogeneic bone marrow transplantation (BMT). A woman with chronic myeloid leukaemia received an allogeneic BMT from a human leucocyte antigen-identical brother. The donor, a 50-year-old non-smoker, died of squamous cell bronchogenic carcinoma 1 year later. At 4 years post BMT, the patient became BCR/ABL positive and relapsed with acute myeloid leukaemia, which was shown to be donor-derived cytogenetically and molecularly. Retrospective analysis showed that the donor-leukaemic clone had started to evolve as early as 6 months post BMT. Sequencing of p53 ruled out Li-Fraumeni syndrome. Predisposition to malignancy might be an underlying mechanism of donor-cell leukaemia.