Erythropoietin is a polypeptide hormone, which is produced in response to a deficit in the delivery of oxygen to the tissues relative to their oxygen needs; although other feedback mechanisms also modulate its production. The kidneys play a major role in the elaboration of this hormone. However, the precise action of the kidney is not yet known. Diseases of the kidneys cause profound alteration in the normal regulation of erythropoiesis. Some of these are reviewed here.