Forty-four patients with myelofibrosis were investigated in our hospital in the period 1971-81. Their clinical, laboratory and radioisotope parameters were analysed. The direct correlation between plasma volume and splenic red cell pool has highlighted the role of the spleen in the dilutional anaemia seen in myelofibrosis. 52Fe quantitation enabled us to show that the bone marrow contributes relatively more to effective erythropoiesis than the extramedullary sites. The prognostic value of changes in plasma volume and bone marrow 52Fe activity has been demonstrated. We have shown that the Hb: reticulocyte relationship at diagnosis can be used to recognize probable stages of the disease and provides a useful prognostic determinant.