The erythropoietic part of the bone marrow has been morphologically analysed in 34 untreated, anaemic patients with acute myeloid leukaemia and in 17 healthy controls. In the patients with leukaemia the percentage of basophilic erythroblasts was abnormally high and elevated mitotic indices of the erythroblasts were recorded. The accumulation of basophilic erythroblasts and a high frequency of megaloblastoid changes of the cells may indicate ineffective erythropoiesis. A threefold increase of "erythroblastic islands", i.e. erythroblasts in contact with reticulum cells were recorded in the patients with erythroleukaemia and a twofold increase in the patients with acute myeloid leukaemia. The erythroblastic islands may indicate phagocytosis of defective erythroblasts and an intramedullary haemolysis may thus contribute to the development of anaemia.