In addition to permanent weight loss various complications are associated with bariatric surgery. This case report describes two women who developed pronounced anaemia after Roux-en-Y gastric bypass (RYGB), which had to be treated with intravenous iron substitution. Anaemia due to reduced iron uptake is common after RYGB and menstruating women are at particular risk. The two cases stress the importance of compliance to the recommendations of iron substitution and routine controls, and the importance of reducing the menstrual bleeding.