3 patients with myelofibrosis with myeloid metaplasia were splenectomized because of anaemia and disturbing splenomegaly. In the course of the 6 months following splenectomy, a polycythaemia developed. Erythrokinetic studies demonstrated that in all cases a reduction in plasma volume and an increase in red cell volume was obtained. Total erythropoiesis decreased along with normalization of ineffective erythropoiesis and peripheral haemolysis. The reappearance of an erythropoietic activity measured over the sacrum was a constant finding, while in 1 patient, a depression of activity over the liver was observed. The new distribution and organization of erythropoiesis in the splenectomized patients is hypothesized as being due to the removal of the influence of an enlarged spleen on erythropoietic organs.