Purpose: RYGB is the most common surgical procedure performed for medically-complicated obesity. We previously reported a high prevalence of thiamine deficiency associated with small intestinal bacterial overgrowth (SIBO) after RYGB (Nutrition Res, 2008). Clinical manifestations of Beriberi include: 1) Neuropsychiatric; 2) High output cardiovascular disease (wet Beriberi); 3) Neurological/Neuropathies (dry Beriberi); or 4) Gastrointestinal (nausea/emesis/megajejunum or constipation/megacolon). The purpose of this study is to estimate the prevalence of and subtypes of Beriberi after RYGB. Methods: Prospective study performed in an urban community hospital. Consecutive RYGB patients seen at a new joint Medical/Surgical Bariatric Clinic from February 1, 2008 to May 1, 2009 were included in this study. Data included date of and type (open or laparoscopic) of RYGB, sex, age, thiamine diphosphate levels, Beriberi symptoms (Types 1-4), thiamine supplementation, and potential resolution with thiamine treatment. Beriberi is defined in this study by: 1) consistent clinical symptoms and examination and 2) either low blood thiamine level (n=22), or resolution of consistent clinical symptoms after intramuscular (IM) thiamine (n=5). Results: Of 151 patients evaluated, 25 females and 2 males met the criteria for Beriberi (estimated prevalence of 18%); age range is 30-68 years (mean age: 50 years); 4 patients had laparoscopic while 23 had open RYGB. Symptoms were noted 2 months to 9 years (mean: 5.0 years) after RYGB. In these 27 RYGB patients, 12 have one symptom consistent with Beriberi while 15 have symptoms consistent with multiple subtypes. Among these patients, 52% had Type 1 Beriberi, 67% had Type 2, 11% had Type 3, and 48% had Type 4 (GI) Beriberi. Of 20 patients with symptoms of Beriberi prior to treatment with IM thiamine, all 20 (100%) noted post-treatment symptomatic improvement. In 3 patients, a symptom of Beriberi returned after replacing IM thiamine therapy with oral thiamine supplements. Conclusion: In our patient population, a high prevalence of Beriberi was identified on average 5.0 years after RYGB. The most common Beriberi identified was Type 2 (wet Beriberi). General gastroenterologists are seeing an increasing number of bariatric patients. For symptoms of nausea, emesis or constipation, GI Beriberi should be considered. However, other types of Beriberi may be present. Oral thiamine alone may be an ineffective treatment in these patients, supporting malabsorption. If there is a clinical suspicion of Beriberi in a RYGB patient, therapy with IM thiamine should be started. Further studies are required to determine whether treating SIBO in RYGB patient prevents Beriberi or may be effective as an adjuvant therapy in patients with Beriberi.