Erythropoietin is a glycoprotein hormone of primarily renal origin that promotes the proliferation and differentiation of erythrocyte precursors. Technological advances have resulted in the production of recombinant hormone suitable for therapeutic use and have permitted significant progress in the characterization of the physiologic and pathologic processes involved in endogenous erythropoietin production. In situ hybridization studies have shown that erythropoietin production in the hypoxic kidney occurs primarily in peritubular cells, most likely endothelial cells. In renal carcinoma associated with polycythemia, however, erythropoietin mRNA has been detected in the tumor cells, which are tubular in origin. New information regarding the biochemistry of the erythropoietin receptor has been gleaned subsequent to the cloning of the gene encoding the receptor; however, much remains to be learned about the interaction of the hormone with its target cells. With regard to clinical experience, recombinant erythropoietin has been shown to correct the anemia associated with chronic renal failure in patients requiring dialysis, having a significant beneficial effect on the overall physical and psychological state of the patient; the major adverse effect of such treatment is hypertension. The role of recombinant erythropoietin in predialysis patients, patients with anemias of other origin, and other clinical settings is currently being evaluated.