Beriberi is primarily related to malnutrition, but also known as a complication following abdominal surgical procedures. A 41 year old woman, who underwent gastric bypass surgery 10 years earlier, presented at the emergency department of Landspitali with general weakness and paresthesia. Physical examination revealed muscle atrophy and attenuated deep tendon reflexes in her lower limbs; presenting as polyneuropathy due to beriberi. The diagnosis was confirmed with serum thiamine levels measuring well below reference levels. She was treated with 300 mg of thiamine intravenously and her condition started to improve. Thiamine deficiency in this case is most likely related to malnutrition secondary to the gastric bypass surgery. This rare complication must be considered in patients that present with polyneuropathy following bariatric surgery. Key words: Beriberi, gastric bypass, thiamine, polyneuropathy, surgery, case report. Correspondence: Tomas Gudbjartsson, tomasgud@landspitali.is.