Iron deficiency is an important cause of anemia in patients with chronic kidney disease (CKD), but intravenous iron is infrequently used among patients who are not on dialysis. Ferumoxytol is a novel intravenous iron product that can be administered as a rapid injection. This Phase III trial randomly assigned 304 patients with CKD in a 3:1 ratio to two 510-mg doses of intravenous ferumoxytol within 5 +/- 3 d or 200 mg of elemental oral iron daily for 21 d. The increase in hemoglobin at day 35, the primary efficacy end point, was 0.82 +/- 1.24 g/dl with ferumoxytol and 0.16 +/- 1.02 g/dl with oral iron (P < 0.0001). Among patients who were not receiving erythropoiesis-stimulating agents, hemoglobin increased 0.62 +/- 1.02 g/dl with ferumoxytol and 0.13 +/- 0.93 g/dl with oral iron. Among patients who were receiving erythropoiesis-stimulating agents, hemoglobin increased 1.16 +/- 1.49 g/dl with ferumoxytol and 0.19 +/- 1.14 g/dl with oral iron. Treatment-related adverse events occurred in 10.6% of patients who were treated with ferumoxytol and 24.0% of those who were treated with oral iron; none was serious. In summary, a regimen of two doses of 510 mg of intravenous ferumoxytol administered rapidly within 5 +/- 3 d was well tolerated and had the intended therapeutic effect. This regimen may offer a new, efficient option to treat iron deficiency anemia in patients with CKD.