Abdominal scintiscans were performed and three-day fecal indium-111 radioactivity measured, following injection of indium-111-labeled polymorphonuclear leukocytes, in patients with acute gastroenteritis, enteropathogen carriage, exacerbations of chronic inflammatory bowel disease, and patients without gastrointestinal symptoms. The colon was more commonly inflamed than the small intestine in acute gastroenteritis. Fecal indium-111 radioactivity excretion was elevated in gastroenteritis and in chronic inflammatory bowel disease. The magnitude of the intestinal inflammatory response, as measured by fecal indium-111 excretion, is equivalent in acute gastroenteritis caused by a defined enteropathogen and exacerbations of chronic inflammatory bowel disease. All patients with microscopically detected fecal leukocytosis gave positive intestinal scintiscans, whereas negative scans were obtained on patients without fecal leukocytosis. The results of this study suggest that indium-111-labeled polymorphonuclear leukocytes can be used to study pathophysiology of the enteric inflammatory response in acute infectious gastroenteritis.