OBJECTIVES: Anaemia is a frequent complication after cardiopulmonary bypass surgery. Iron therapy has been variably employed by medical centres over the years. In our study we test the clinical effectiveness of intravenous and oral iron supplementation in correcting anaemia, and its impact on blood transfusion requirements, in patients undergoing cardiopulmonary bypass surgery. METHODS: A double-blind, randomized, placebo-controlled clinical trial with three parallel groups of patients. Group I (n = 54): intravenous iron(III)-hydroxide sucrose complex, three doses of 100 mg/24 h during pre- and postoperative hospitalization and 1 pill/24 h of oral placebo in the same period and during 1 month after discharge. Group II (n = 53): oral ferrous fumarate iron 1 pill/24 h pre- and postoperatively and during 1 month after discharge, and intravenous placebo while hospitalized. Group III (n = 52): oral and intravenous placebo pre- and postoperatively, following the same protocol. Data were collected preoperatively, at theatre, at intensive care unit admission, before hospital discharge and 1 month later. RESULTS: (1) Baseline clinical and demographic characteristics and surgical procedures were similar in the three groups; (2) no inter-group differences were found in haemoglobin and haematocrit during the postoperative period; (3) the intravenous iron group showed higher serum ferritin levels at hospital discharge (1321 ± 495 ng/ml; P < 0.001) and 1 month later (610 ± 387; P < 0.001) compared with the other groups and (4) we did not observe statistical differences in blood transfusion requirements between the three groups. CONCLUSIONS: The use of intravenous or oral iron supplementation proved ineffective in correcting anaemia after cardiopulmonary bypass and did not reduce blood transfusion requirements. [Current Controlled Trials number: NCT01078818 (oral and intravenous iron in patients postoperative cardiovascular surgery under EC)].