Thirty-two children with extramedullary myeloid cell tumour (EMT) who constituted 41 per cent of children with acute myeloid leukaemia (AML) were studied to ascertain their laboratory characteristics and potential problems in diagnosis. The diagnosis, established by peripheral blood smear and/or bone marrow examination, was AML (n = 29) and refractory anaemia with excess blasts in transformation (RAEB-t; n = 3). The six referred patients in whom the diagnosis had been missed, and two cases wrongly reported as histiocytosis on aspiration cytology, were those in whom a peripheral blood smear had not been examined. It is concluded that diagnostic work-up of proptosis must include a full haemogram, meticulous peripheral blood smear examination, repeated if necessary, and bone marrow examination where relevant. RAEB-t cases with extramedullary myeloid cell tumour should be classified as acute myeloid leukaemia.