Serum erythropoietin (EPO) levels were measured by radioimmunoassay in 36 patients undergoing allogeneic bone marrow transplantation (BMT). Serum EPO levels before conditioning treatment for BMT were generally higher than the levels obtained from healthy controls (49 +/- 17 (SEM) and 17 +/- 0.6, respectively). One day prior to BMT, after conditioning by chemotherapy with or without total body irradiation, the mean EPO level was markedly elevated (218 +/- 23 U/l, p less than 0.001) and reached to its highest level at 1 week post-BMT (269 +/- 40 U/l). Although, the EPO levels were significantly lower at 1 month (98 +/- 24 U/l, p less than 0.001), they were still elevated up to 3 months post-BMT, after which they gradually normalized. Patients given methotrexate and cyclosporine for prophylaxis against graft-versus-host disease (GVHD) had significantly lower EPO levels during the first 3 months post-BMT than patients transplanted with T cell-depleted marrow (p less than 0.05). Patients with post-transplant nephrotoxicity had lower, though not statistically significant, EPO levels than patients with normal renal function (p = 0.07). Acute GVHD and number of blood transfusions had no influence on serum EPO levels after BMT.