Erythropoietin (Epo), the first growth factor to be discovered, is an endocrine hormone produced by specialized renal cells. The rate of Epo production is determined primarily by the oxygen demands of these renal cells relative to their oxygen supply. However, Epo production is modulated by various hormones, nutritional factors, cytokines, and the integrity of the erythron. Epo interacts with specific receptors found almost exclusively on erythroid progenitors. This interaction results in an expansion of the number of the erythroid progenitor and triggers late committed progenitors to undergo terminal maturation when provided with essential nutrients. Recombinant human Epo (rHuEpo) is commercially available for human use. It is safe, easily administered, and almost universally effective in treating the anemia of patients with renal failure. It has also been successful in treating the anemia of some patients with neoplasms, myelodysplastic syndromes, HIV infection, rheumatoid arthritis, and aplastic anemia. Much remains to be learned about the regulation of Epo production, the physiologic actions of Epo, and how best to use this growth factor in the treatment of anemia.