The term familial and congenital polycythemia encompasses a heterogeneous group of disorders with the common characteristic of an absolute increased red cell mass since birth and/or similar phenotype also present in relatives. In the last 2 decades the differential diagnosis between primary and secondary familial polycythemias became more physiologically relevant as new sensitive techniques, such as accurate measurements of serum erythropoietin (S-EPO) concentration by radioimmunoassay (RIA) or ELISA, and assessment of growth of erythroid progenitor cells in vitro became available. Consequently, correct classification of many older previous reports of familial polycythemias is difficult. While familial secondary polycythemias due to high oxygen affinity hemoglobin mutants are not infrequent and have been well delineated in terms of molecular pathophysiology and phenotype during the last 3 decades, those secondary familial polycythemias due to 2,3 DPG deficiency are very rare. Familial and congenital polycythemias with increased EPO concentration and normal arterial oxygen saturation and oxygen dissociation kinetics represent an intriguing group of disorders wherein the molecular lesions remain obscure; however, in some instances a possibility of abnormal oxygen sensing pathway involving hypoxia inducible factor-1 (HIF-1) open an intriguing yet unexplored area of hematology and biology. In contrast the primary familial and congenital polycythemia (PFCP) has been only recently recognized (the first report published in 1977). Various designations have been used in the past to describe PFCP, a rare clinical syndrome, including: benign familial erythrocytosis, polycythemia vera of childhood, primary polycythemia, pure erythrocytosis, etc. Some of these terms stressed the relatively benign, non-progressive course of the disease with a normal lifespan of affected subjects; however, the apparent benignity of some of these disorders has been questioned. These disorders are familial and/or congenital, and the clinical and laboratory evidence of secondary polycythemias must be excluded. Only about 2 dozen familial and sporadic cases with PFCP have been reported. However, the mutations of erythropoietin receptor (EPOR) found in some of families with PFCP represent the only defined molecular defect of primary polycythemic phenotypes. All reported PFCP associated EPOR mutations result in truncation of its intracytoplasmic C-terminal domain which negatively regulates the EPO/EPOR signal transduction pathway. Subjects with these mutations have decreased or normal S-EPO and increased sensitivity of erythroid progenitor cells to low EPO concentrations in in vitro assays. Mutations of other genes involved in post EPOR signaling pathway such as JAK-2, HCP and STAT 5 may also play a causative role in pathogenesis of some of PFCP families where mutation of EPOR was not found.