An 83-year-old Caucasian man presented with severe, watery diarrhea of 3 days' duration that was associated with nausea and dull, diffuse abdominal pain. There was no fever, chills, or history of recent antibiotic use. His past medical history included non-insulin-dependent diabetes mellitus, chronic renal failure, mild dementia, and an inhibitor to factor 8 requiring rituximab and prednisone 6 years earlier. He required hospitalization because of severe dehydration causing acute on chronic renal failure. After treatment with intravenous (IV) fluids, his renal function and stooling improved, and he was discharged on the third hospital day.The day after discharge, he again presented with diarrhea and acute renal failure. He continued to have profuse watery diarrhea despite being given nothing by mouth. In addition to IV fluids, total parenteral nutrition was started. Stool for culture, ova and parasites, Clostridium difficile, and Giardia was negative. Colonoscopy showed focal colitis without evidence of inflammatory bowel disease or ischemic colitis. After 7 days of persistent, large-volume diarrhea, antibiotics with ciprofloxacin and then metronidazole were given because of the suspicion of bacterial overgrowth. The diarrhea improved but did not resolve.An esophagogastroduodenoscopy was then performed that showed villous atrophy. The periodic acid-Schiff stain for Tropheryma Whippelii was negative. Serum immunoglobulin (Ig) G tissue transglutaminase (tTG) antibodies were negative. He was managed with a gluten-free diet, and his diarrhea gradually resolved. It was unclear whether he had small bowel bacterial overgrowth or celiac disease. Because his tTG antibodies were negative, and his diarrhea seemed to be improving with antibiotics, his clinical picture was attributed to bacterial overgrowth, and a regular diet was resumed.When his diet was liberalized to include glutencontaining food, he developed diarrhea requiring hospitalization. Additional studies from the small intestinal biopsy were positive for CD31 lymphocytes in the mucosa. IgA antiendomysial antibodies were positive. He responded to a gluten-free diet but required a bile acid sequestrant to control the diarrhea.