AIM: To evaluate the incidence and risk factors for the development of anemia after Roux-en-Y gastric bypass (RYGB). METHODS: A retrospective analysis of patients undergoing RYGB from January 2003 to November 2007 was performed. All patients had a preoperative body mass index > 40 kg/m~2. A total of 206 patients were evaluated. All patients were given daily supplements of ferrous sulfate tablets for 2 wk following their operation. Hematological and metabolic indices were routinely evaluated following surgery. Patients were followed for a minimum of 86 wk. RESULTS: There were 41 males and 165 females with an average age of 40.8 years. 21 patients (10.2%) developed post-operative anemia and 185 patients (89.8%) did not. Anemia was due to iron deficiency in all cases. The groups had similar demographics, surgical procedure and co-morbidities. Menstruation ( P = 0.02) and peptic ulcer disease ( P = 0.01) were ris k factors for the development of post-operative anemia. CONCLUSION: Iron deficiency anemia is frequent. RYGB surgery compounds occult blood loss. Increased ferrous sulfate supplementation may prevent iron depletion in populations at increased risk.