Authors have reviewed recent literature on erythropoietin. They report new information regarding synthesis, modulation, changes during several diseases and the possibility for a clinical use of erythropoietin for the correction of many anaemias. Finally the Authors report some personal observations about the physiopathology of erythropoietin. They have studied the circadian rhythm, action of aging and the prostaglandin system, and plasma levels of E. in patients with anaemia of active rheumatoid arthritis.