Summary We report the case of a patient who, over a 20-month period, developed overt B-cell non-Hodgkin's lymphoma in a T-cell rich background initially indistinguishable from reactive lymphoid aggregates in bone marrow. This morphological pattern of B cell lymphoma has been termed T-cell-rich B-cell lymphoma and, to our knowledge, has not previously been reported with a primary bone marrow presentation. Of additional interest, the patient's initial presentation was with myelodysplasia which appeared morphologically to resolve as the lymphomatous population emerged. Subsequently, however, the patient developed overt acute myeloid leukaemia (AML) and demised. This patient appeared to have two distinct neoplastic processes occurring in his bone marrow. The relationship between the two remains a matter of speculation.