Background.Iron deficiency is a common cause of anaemia and hyporesponsiveness to erythropoiesis-stimulating agents (ESAs) in non-dialysis-dependent chronic kidney disease (ND-CKD) patients.Current intravenous iron agents cannot be administered in a single high dose because of adverse effects.Ferric carboxymaltose, a non-dextran parenteral iron preparation, can be rapidly administered in high doses.Methods.This open-label trial randomized 255 subjects with glomerular filtration rates ≤45 mL/min/1.73 m2,haemoglobin ≤11 g/dL, transferrin saturation ≤25%,ferritin ≤300 ng/mL, and stable ESA dose to either intravenous ferric carboxymaltose 1000 mg over 15 min (with up to two additional doses of 500 mg at 2-week intervals) or oral ferrous sulphate 325 mg thrice daily for a total of 195 mg elemental iron daily for 56 days.Results.In the modified intent-to-treat population, the proportion of subjects achieving a haemoglobin increase ≥ 1 g/dL at any time was 60.4% with ferric carboxymaltose and 34.7% with oral iron (P < 0.001).At Day 42, mean increase in haemoglobin was 0.95 ± 1.12 vs 0.50 ±1.23 g/dL (P =0.005), mean increase in ferritin vas 432 ±189 ng/mL vs 18 ± 45 ng/mL (P < 0.001) and mean increase in transferrin saturation was 13.6 ± 11.9% vs 6.1 ± 8.1%(P < 0.001).Treatment-related adverse events were significantly fewer with ferric carboxymaltose than with oral iron (2.7% and 26.2%, respectively; P < 0.0001).Conclusions.We conclude that 1000 mg ferric carboxymaltose can be rapidly administered, is more effective and is better tolerated than oral iron for treatment of iron deficiency in ND-CKD patients.