Three patients with megakaryoblastic leukaemia are described. All three presented with pancytopenia, a few blast cells in the peripheral blood and absence of overt hepatosplenomegaly. In two of them bone marrow aspiration yielded a dry tap. Histological investigation of the bone marrow indicated that the megakaryocytic cell line was the dominant proliferating lineage. Cytochemical and EM investigation supported these findings. The isomorphic isoenzyme pattern of the elevated serum lactic dehydrogenase might be of diagnostic importance. Despite chemotherapy, there was a rapidly fatal terminal leukaemic phase with high blast cell counts. The differentiation from other haematological malignancies, especially acute (aleukaemic) leukaemias and the accelerated phase of primary (chronic) myelofibrosis, is discussed. The picture appears to be identical with acute (malignant) myelofibrosis.