In successful renal transplant recipients, transient and modest increases in endogenous erythropoietin (Epo) reverse anemia, whereas in dialysis patients, sustained administration of large doses of exogenous Epo is required for the correction of uremic anemia. Moreover, in transplant recipients, serum Epo returns to normal as the hematocrit level increases to greater than 32%. Thereafter, the hematocrit continues to increase to normal levels, while serum Epo remains in the normal range. Thus, the restoration of renal function may improve the erythropoietic response to Epo, and/or erythropoiesis in transplant patients may be stimulated by factors other than, or in addition to, Epo. In early posttransplant patients who develop erythrocytosis, serum Epo levels are often elevated, while in long-term transplant recipients, erythrocytotic patients (with normal serum ferritin) have normal serum Epo levels. On the other hand, in long-term transplant recipients with low serum ferritin, circulating Epo levels are elevated, even in patients with no overt anemia. This suggests a possible interaction between body iron store status and the synthesis of Epo.