The recurrence of leukaemia following allogeneic bone marrow transplantation appears to develop rarely in donor cells. However, the standard method for assigning the origin of recurrence, metaphase analysis, can be unreliable. We have applied the technique of fluorescence in situ hybridization (FISH) directly on archival Wright stained bone marrow slides obtained from a patient who developed acute myelogenous leukaemia (AML) following allogeneic bone marrow transplantation (BMT) for chronic myelogenous leukaemia (CML). Using a chromosome-specific DNA probe we linked a chromosomal aberration, previously detected by conventional metaphase analysis, directly to morphologically identifiable blast cells. In this way we were able to assess cell-lineage involvement of the secondary leukaemia and assign a donor origin.