I have examined Ferrokinetics and the activity of serum erythropoietin (by Frieds method) for determining the in vivo erythropoiesis, additionally the presence of "effective" or "ineffective" erythropoiesis in patients with hypoplastic anemia. The number of reticulocytes and the reticulocyte production index also were analysed concerning erythropoiesis. Based on above-mentioned findings, the prognosis of these patients was analysed. Furthermore, from the relationship between the morphological examination of the erythroid cells in the bone marrow and the time course of the activity of serum erythropoietin, the change of erythropoiesis was investigated. From these results, a higher level of serum erythropoietin was observed in patients with more severe type in erythropoiesis, whereas the prognosis in these patients was much poorer in relation to an elevated value of serum erythropoietin. Furthermore, it might be suggested that a high level of erythropoietin which is enough to help the erythroid progenitor cells to differentiate and maturate to normal erythroblasts was able to shorter the maturation time of the erythroid cells and also effectively abolished "ineffective erythropoiesis" inducing the proper erythropoiesis.