Upper gastrointestinal tract surgery and diuretic use are 2 unrecognized causes of thiamine (vitamin B1) deficiency. Upper gastrointestinal tract surgery decreases the thiamine absorption, and diuretic use increases urinary excretion of thiamine. We present a case of a patient with a history of pancreaticoduodenectomy who had development of beriberi by diuretic use. A 68-year-old man was referred to our hospital because of pretibial pitting edema, foot numbness, and gait disturbance. He had a history of pancreaticoduodenectomy 8 years before and had been taking loop diuretics for 2 months. He had signs of polyneuropathy and hyperkinetic heart. Beriberi was suspected, and thiamine supplementation was started immediately. Edema disappeared within several days, and signs of polyneuropathy gradually subsided. Because diuretics enhance urinary thiamine excretion, practitioners should use caution for thiamine deficiency when they prescribe diuretics for patients who have a history of upper gastrointestinal surgery and potentially have latent thiamine deficiency.