Background Laparoscopic Roux-en-Y gastric bypass (LRYGB) can lead to iron malabsorption through exclusion of the duodenum and proximal jejunum, decreased gastric acidity, and modified diet. Intravenous (IV) iron is a treatment for severe iron deficiency, but the incidence of iron deficiency and the frequency of treatment with IV iron after LRYGB are largely unknown. Our objective was to determine the incidence of iron deficiency and the frequency of IV iron administration after LRYGB. Study Design After obtaining IRB approval, the medical records of patients who underwent LRYGB from September 2001 to December 2011 were retrospectively reviewed. Inclusion criteria consisted of determination of at least 1 ferritin value after surgery. Patients were grouped by level of iron deficiency. Patients with at least 1 ferritin <50 ng/mL were considered iron deficient. Statistical analysis included ANOVA. Results There were 959 patients included; 84.9% were female. Mean age was 43.8 years, and preoperative body mass index was 47.4 kg/m2. Four hundred ninety-two (51.3%) patients were iron deficient. Of these, 40.9% were severely iron deficient, with a ferritin <30 ng/mL. Intravenous iron was required by 6.7%. After IV iron therapy, 53% had improvement in hemoglobin and ferritin values, and 39% had improvement in ferritin values only. Conclusions Given the incidence of iron deficiency after LRYGB observed in our series, patients should have iron status monitored carefully by all providers and be appropriately referred for treatment. Female patients should be counseled that there is a 50% chance they will become iron deficient after LRYGB.