OBJECTIVE: To evaluate the efficacy of IV iron supplementation of anemic, critically ill trauma patients. DESIGN: Multicenter, randomized, single-blind, placebo-controlled trial. SETTING: Four trauma ICUs. PATIENTS: Anemic (hemoglobin < 12 g/dL) trauma patients enrolled within 72 hours of ICU admission and with an expected ICU length of stay of more than or equal to 5 days. INTERVENTIONS: Randomization to iron sucrose 100 mg IV or placebo thrice weekly for up to 2 weeks. MEASUREMENTS AND MAIN RESULTS: A total of 150 patients were enrolled. Baseline iron markers were consistent with functional iron deficiency: 134 patients (89.3%) were hypoferremic, 51 (34.0%) were hyperferritinemic, and 64 (42.7%) demonstrated iron-deficient erythropoiesis as evidenced by an elevated erythrocyte zinc protoporphyrin concentration. The median baseline transferrin saturation was 8% (range, 2-58%). In the subgroup of patients who received all six doses of study drug (n = 57), the serum ferritin concentration increased significantly for the iron as compared with placebo group on both day 7 (808.0 ng/mL vs 457.0 ng/mL, respectively, p < 0.01) and day 14 (1,046.0 ng/mL vs 551.5 ng/mL, respectively, p < 0.01). There was no significant difference between groups in transferrin saturation, erythrocyte zinc protoporphyrin concentration, hemoglobin concentration, or packed RBC transfusion requirement. There was no significant difference between groups in the risk of infection, length of stay, or mortality. CONCLUSIONS: Iron supplementation increased the serum ferritin concentration significantly, but it had no discernible effect on transferrin saturation, iron-deficient erythropoiesis, hemoglobin concentration, or packed RBC transfusion requirement. Based on these data, routine IV iron supplementation of anemic, critically ill trauma patients cannot be recommended (NCT 01180894).