Objective. Patients with inflammatory bowel disease (IBD) often have low iron stores or anaemia. There is controversy about whether iron should be supplemented orally or intravenously (i.v.). The purpose of this study was to investigate whether treatment with intravenous iron is superior to treatment with oral iron. The primary end-points were response and remaining anaemia at the end of treatment (EOT). Material and methods. Ninety-one patients with IBD and anaemia (B-Hb <115 g/L) were randomized to oral iron sulphate (n=46) or intravenous iron sucrose (n=45) treatment for 20 weeks. Results. Forty-three patients in the intravenous iron group completed the study compared to 35 patients in the oral iron group (p=0.0009). Only 22 patients (48%) tolerated the prescribed oral dose, and 52% reduced the dose or withdrew from treatment because of poor tolerance. At EOT, 47% patients in the oral iron group increased their B-Hb by ≥20 g/L, compared with 66% in the intravenous iron group (p=0.07). In the oral iron group, 41% still had anaemia versus 16% of the patients in the intravenous iron group (p=0.007), and 22% versus 42% reached their reference B-Hb level (p=0.04). Treatment with intravenous iron sucrose improved iron stores faster and more effectively than oral iron (p=0.002). Under treatment with intravenous iron, 74% of the patients had no anaemia and normal S-ferritin levels (>25 µg/L) at EOT compared with 48% of patients receiving oral iron (p=0.013). Conclusions. Treatment with intravenous iron sucrose is effective, safe, well tolerated and superior to oral iron in correcting haemoglobin and iron stores in patients with IBD.