Anemia affects over half of the 10 million Americans with pre-dialysis chronic kidney disease (CKD) and is associated with a variety of adverse effects including mortality and decreased quality of life. Anemia which does not respond to treatment with erythropoiesis stimulating agents (ESA) is common in clinical practice. Much evidence points to adverse health effects associated with escalation of ESA therapy in CKD patients with treatment resistant anemia. However, little progress has been made toward identifying other mechanisms which contribute to anemia in CKD in order to identify safe and effective adjunctive treatments. Hepcidin, an anti-microbial peptide produced by the liver, whose production is stimulated by inflammation, interferes with the utilization of iron stores for erythropoiesis. The specific contribution of hepcidin to the development of anemia in children with CKD has not been explored in a prospective manner. In addition, emerging evidence suggests that 25-hydroxy vitamin D (25D) deficiency is a novel risk factor for anemia in CKD, which may be due to 25D-dependent immunomodulation. Interruption of the inflammatory stimuli which increase hepcidin production could make the management of CKD-associated anemia with iron supplementation and ESAs more effective. The long-term goal of the proposed research is to define the contribution of hepcidin mediated mechanisms to the anemia of early CKD, and to understand whether hepcidin production may be modified by a therapeutic intervention. The Specific Aims of this project are to (1) define how elevated serum levels of hepcidin are associated with hemoglobin and ESA hypo-responsiveness within the established NIH funded Chronic Kidney Disease in Children (CKiD) prospective cohort study, and (2) to evaluate the effects of ergocalciferol (vitamin D2) supplementation on serum levels of hepcidin, hemoglobin, and ESA dose in a prospective interventional trial in children with pre-dialysis CKD. Our central hypothesis is that in mild to moderate CKD, elevated serum hepcidin is associated with anemia (as defined by low hemoglobin). Furthermore, we hypothesize that in the setting of CKD, supplementation with ergocalciferol will be associated with a decrease in serum hepcidin and increase in hemoglobin. Our hypotheses have been formulated on the basis of an extensive review of the literature on vitamin D, hepcidin and anemia, and our own preliminary data produced by measuring serum hepcidin values in 124 children enrolled in CKiD, in whom higher hepdidin levels are associated with anemia, and preliminary study of normal children showing low 25D levels associated with anemia. The rationale for the combination of observational and interventional approaches proposed is to accelerate our understanding of the biologic mechanisms contributing to the anemia of CKD and simultaneously to develop new and innovative interventions to treat this common co-morbidity in childhood CKD.