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Eighty-three patients with myelofibrosis have been studied by erythrokinetics and have been followed up until death or for at least 12 months. Because of a large plasma volume the venous haematocrit gives only a poor idea of the red blood cell volume. The red cell survival was reduced in the majority of cases but significant haemolysis was rare. The amount of haemolysis of autologous and isologous red cells was similar, suggesting an extra-corpuscular origin for the haemolysis. Plasma iron turnover was always increased, sometimes markedly, but red cell iron incorporation was reduced in 70% of cases, indicating ineffective erythropoiesis. Surface counting showed an absence on diminution of sacral iron fixation and a rapid and marked splenic uptake in more than 90% of the cases; uptake of iron by the liver occurred in half the cases, usually not very high; iron release from the spleen was absent or reduced in 67% of the cases. The degree of ineffective erythropoiesis as measured by radio-iron incorporation and release by the spleen, the amount of haemolysis, and the red cell volume were strongly correlated with prognosis. These factors enabled a more precise prediction to be made of the clinical outcome in the 2 years following the study, than the clinical data alone. A prospective study might show whether erythrokinetic studies are also useful in determining the choice of treatment.