This review will discuss evidence for the role of the erythropoietin (Epo) receptor in the development of erythrocytosis and other hematological disorders. The possible causative role of mutations of other genes in the pathogenesis of idiopathic erythrocytosis will be considered. Polycythemia vera (PV) is a myeloproliferative disorder that is caused by an undefined stem cell abnormality, characterized by a significant erythrocytosis, leukocytosis, and thrombocytosis. However, erythrocytosis may arise from apparent (or relative) polycythemia in which the hematocrit is raised due to a low plasma volume. In such cases the red cell mass is normal. A group of disorders with increased red cell mass caused by stimulation of erythrocyte production is known as secondary polycythemia. Investigation of such patients may reveal a congenital abnormality such as high affinity hemoglobin or an acquired abnormality caused, for example, by smoking, renal vascular impairment, or an Epo-producing tumor. Even after thorough examination there remains a cohort of patients for whom no definite cause for the erythrocytosis can be established. A careful clinical history may reveal whether this idiopathic erythrocytosis is likely to be congenital and/or familial, in which case the term "primary familial and congenital polycythemia" is sometimes applied. Access to a range of laboratory investigations may define the molecular pathophysiology. We will now discuss how this process can be investigated.