In an attempt to stimulate endogenous erythrocyte production and thereby provide an alternative to erythrocyte transfusions, we administered recombinant human erythropoietin (rHuEpo) in doses of 75 to 300 units/kg/wk to seven infants with the anemia of prematurity. Treatment was started between 21 and 33 days of life, maintained for 4 weeks, and was well tolerated. All the patients had low baseline serum erythropoietin levels. After rHuEpo therapy, the number of reticulocytes increased from a mean baseline count of 75 x 10(9)/L to 95, 141, and 165 x 10(9)/L on days 7, 10, and 14 of therapy, respectively. Correction or stabilization of the anemia was observed in six of seven patients, whose estimated total erythrocyte volume increased by 49% during therapy (vs a predicted increment of 18% in the absence of rHuEpo). In one patient, however, the hematocrit declined during the treatment, and in three of the responders a secondary fall in hematocrit was noted either during therapy or after its discontinuation. Serum iron and ferritin levels rapidly decreased after the initiation of rHuEpo therapy, and in most patients transient early thrombocytosis and late neutropenia were observed. These data suggest that rHuEpo may correct or stabilize the anemia of prematurity. Its effects, however, may be limited by a variety of factors, among which iron availability probably plays an important role. Controlled studies will be needed to confirm these preliminary observations.