Treatment of severe obesity with Roux-en-Y gastric bypass (RYGB) results in pronounced weight loss but also in various nutritional complications. Iron deficiency anaemia is one of the most common nutritional complications due to reduced uptake of iron after this operation. Premenopausal women are particularly at risk of developing iron deficiency anaemia because of menstruation. In the present article the pathophysiology, diagnostic considerations, and treatment of iron deficiency anaemia after RYGB are discussed.