A series of 256 bone marrow biopsy specimens was obtained at different times from a group of patients with acute myeloid leukaemia, chronic granulocytic leukaemia and acute lymphoblastic leukaemia, and was analysed in parallel with peripheral blood smears and bone marrow aspirate samples. In five of these biopsy specimens the bone marrow lesion was discrete, deeply seated within the marrow cavity and detected only in long-core biopsies. Neither peripheral blood nor bone marrow aspirates obtained at the same time established the correct diagnosis. This suggests that when peripheral blood and bone marrow aspirate samples fail to indicate the diagnosis a long-core biopsy may yield positive results.