Recurrent leukaemia following bone marrow transplantation is most often due to the regrowth of original host leukaemia cells, but may also be due to the malignant transformation of normal donor marrow cells after transplantation into a leukaemia patient. We report the ninth case of malignant change in cells of donor origin in a 12-year-old boy who was originally diagnosed as having Ph1+ CML. He remained Ph1+ during lymphoid blast crisis. After transplantation with marrow from a cytogenetically normal sister, he relapsed to Ph1- ALL in the female donor cells. The marrow showed a mixed karyotype of 46,XX/46,XX,inv(9)(p12q12). It would appear that, haematologically, the patient showed different manifestations of the same disease state. Cytogenetically, however, the pre- and post-transplant leukaemias were different.