Thirty-seven consecutive adult patients with a history of adverse reactions to foods, manifested mainly as diarrhea, were investigated with skin prick test (SPT), IgE levels in serum, and the presence of IgE bound to mast cells in duodenal biopsy specimens. Nineteen percent had increased serum IgE levels indicating the presence of atopic disease and in 35% positive SPTs for one or several allergens related to their gastrointestinal symptoms were found. In 92% of the patients with positive SPTs to food, IgE-positive mast cells in duodenal biopsy specimens were found, twice as many as in patients with negative SPTs (47%). Forty-two percent of normal individuals without any adverse reactions to foods had IgE-bearing mast cells in their intestinal mucosa. The results showed that ‘arming’ of mast cells with IgE locally was relatively prominent but not consistent in the gut of patients with food-related diarrhea suggested to be allergic by positive SPT. Intestinal IgE-mediated hypersensitivity reactions could be of pathogenetic significance in most of these patients. However, since also half of the normal individuals were found to have IgE-positive intestinal mast cells, this phenomenon presumably also reflects a normal physiological defence mechanism.