Recombinant human erythropoietin (rhEpo) was given i.v. at a dose of 50 U/kg/tid to eight patients undergoing an HLA-matched, ABO-compatible bone marrow transplantation (BMT), from day +1 up to day +30. Compared to the data recorded in 13 similar BMT patients who had not received the hormone, the administration of rhEpo resulted in a faster erythroid engraftment: in fact, the time required to reach a stable hematocrit value greater than or equal to 35% decreased from 123.0 to 58.0 days after BMT. Moreover, the number of blood reticulocytes on day +21 was about fourfold greater in the rhEpo group than in the controls, while the number of the most immature, high RNA content reticulocytes (HFR), as determined by a flow cytometric technique, was more than sixfold greater; finally, the recovery time of both total and HFR reticulocytes was significantly reduced by rhEpo. The stimulation of erythroid progenitors also resulted in a reduction in red blood cell (RBC) transfusion requirements: the number of RBC units delivered in the first 30 days following BMT decreased from 8.1 in the controls to 4.0, while the total number of RBC units before transfusion independence was about threefold lower than in the control. Finally, the time of transfusion dependence was significantly shortened by rhEpo. No clinically significant adverse effect directly attributable to rhEpo was recorded. These data suggest that the administration of rhEpo may be beneficial in hastening erythroid engraftment, and possibly in reducing RBC transfusion requirements following BMT.