Resident in Internal Medicine (D.K.C.) and Fellow in Gastroenterology (D.A.S.), Mayo School of Graduate Medical Education, Mayo Clinic, Rochester, MN; Adviser to resident and fellow and Consultant in Gastroenterology and Hepatology, Mayo Clinic, Rochester, MN (S.C.H.). A 60-year-old man was transferred to our hospital with a 5-year history of worsening daily watery, nonbloody, nongreasy diarrhea. One year previously, recurrent vomiting had developed. Over the month preceding the current admission, he experienced 4 episodes of intractable, largevolume diarrhea with abdominal pain, upper and lower extremity muscle cramping, and orthostatic light-headedness. His diarrhea did not improve with fasting, and each of these episodes required hospitalization for intravenous fluid and electrolyte replacement. On the fourth episode, he was transferred to our institution for further diagnostic evaluation. The patient reported no symptomatic correlation to dietary habits and no associated fever, gastrointestinal bleeding, flushing, arthralgia, or rash. He had not experienced fecal incontinence. His medical history was notable for gastroesophageal reflux disease diagnosed 4 years previously and treated with 40 mg of esomeprazole daily. He had no surgical history. His only other medication was lisinopril for hypertension, and he reported no recent medication changes or antibiotic therapy. His family history did not include any gastrointestinal illness or malignant neoplasms, and he did not use tobacco or alcohol. His last screening colonoscopy was within the past 5 years and yielded normal findings. The patient had been stabilized with intravenous fluids before arrival at our institution, and physical examination at presentation revealed a well-nourished man with a body mass index of 26.3 kg/m, normal orthostatic vital signs, and normal physical examination findings, including no evidence of rash, abdominal tenderness, fecal impaction, or other abnormality. The results of laboratory studies including red and white blood cell count, complete metabolic profile, calcium level, liver function tests, total protein, albumin, thyrotropin, and tissue transglutaminase antibody values, erythrocyte sedimentation rate, and C-reactive protein