SUMMARYBackground: In patients with inflammatory bowel disease, oral iron is anecdotally reported to be less effective and less well tolerated than in those without inflammatory bowel disease, and to increase disease activity. Aim: To study prospectively the effects of oral iron in patients with and without inflammatory bowel disease. Methods: Patients with ulcerative colitis, Crohn's disease and non-inflammatory bowel disease controls, all with iron deficiency anaemia, were assessed with symptom diaries, a quality of life questionnaire (Inflammatory Bowel Disease Questionnaire; inflammatory bowel disease patients only) and blood tests to measure iron repletion, disease activity and antioxidant capacity before and after starting 4 weeks of oral iron. In patients with ulcerative colitis, sigmoidoscopic scoring and rectal biopsies for reactive oxygen metabolite production were performed before and after iron therapy.