The so-called pure red cell aplasia or hypoplasia may occur in various instances: 1- Congenital form, usually occuring in childhood; 2-Drug-induced types; 3-The forms associated with thymoma, carcinoma and hypogamma-globulinemia; and 4-As an isolated syndrome. In the etiology of congenital PRCA some immune mechanisms are considered to be important but the role of a genetic factor in producing the syndrome is more acceptable, although the exact pathogenesis of impairment of erythropoiesis is not clear. Disregarding the drug-induced types of the aplasia, acquired PRCA with or without thymoma is produced by immunologic mechanisms. Finding some factors such as antibody against erythroblast nuclei, inhibitor of heme synthesis and recently a cytotoxic factor that are in gamma G-globulin fraction of plasma of PRCA patients support the idea that the syndrome is of immunologic origin. It seems that the thymoma is only a manifestation of impaired immunologic state, and its role in producing the aplasia is a subject of controversy. Besides, we can not say that all cases of PRCA have the same pathogenesis and further investigations are necessary to provide some informations about the pathogenesis of this syndrome.