To investigate the potential role of recombinant human erythropoietin (rhEpo) in patients receiving intensive cytotoxic therapy, we measured the endogenous levels of Epo in 31 patients undergoing bone marrow transplantation (BMT). Seventeen patients underwent allogeneic BMT and 14 underwent autologous BMT. On average, 10 +/- 4 units of red blood cells (RBCs) were transfused per patient. The mean RBC transfusion requirement of the autologous BMT patients was significantly greater than that of the allogeneic recipients (12 +/- 3 v 8 +/- 4, P less than .01), although both groups were maintained at comparable hematocrits. Epo levels were measured by radioimmunoassay (RIA). For each patient, baseline serum Epo levels were determined at time of admission to the hospital. Subsequent samples were collected within 24 hours of completing chemotherapy and/or radiotherapy, and on days 7, 14, and 28, after BMT. Hematocrits (Hcts) were measured daily. All patients had an initial serum creatinine less than or equal to 1.5 mg/dL. Despite considerable differences in absolute Epo levels among individuals, a characteristic pattern was observed. Following admission to the hospital and initiation of cytotoxic therapy, the average Hct decreased and the average Epo level initially increased. The mean serum Epo levels peaked on day 7 post-BMT (284 +/- 190 mU/mL) and fell steadily thereafter. While the average Hcts on day 7 and on day 28 post-BMT were not significantly different (28 +/- 4.6% v 29 +/- 3.3%, respectively), the average serum Epo levels decreased fourfold (P less than .01) during this same period. Moreover, day 28 post-BMT mean Epo levels were inappropriately low (P less than .05) when compared with a reference population with bone marrow failure and normal controls who had not received cytotoxic therapy. We conclude that the endogenous Epo response appears to be blunted during the 3 to 4 weeks immediately post-BMT. Therefore, clinical trials assessing the efficacy of the administration of rhEpo in the treatment of anemias associated with cytotoxic therapy are warranted.