The aim of our study was to determine whether patients suffering from food allergy show any pathologic reactions on the mucosa of the gastrointestinal (GI) tract after allergen contact. For this reason, we included in the study 30 patients whose food-allergic history had been proven through double-blind challenge tests; 20 healthy volunteers also were included as controls. The patients and volunteers underwent standard laboratory investigations and allergy tests with PRICK and RAST. To observe possible mucosal reactions, we applied the proposed allergens via endoscope to the gastric mucosa. Macroscopic reactions were observed blindly by two independent physicians. Biopsies were taken from the challenged areas for histological and histochemical analysis. The examinations included the estimations of tissue histamine concentrations and of mast cell and lymphocyte counts. In all 30 patients, macroscopic reactions (swelling, erosions, bleedings) were observed. Patients with food allergy had, in contrast to healthy volunteers, elevated lymphocyte counts, tissue histamine concentrations, and mast cell counts. After provocation, tissue histamine concentrations and mast cell counts fell significantly. Skin and RAST tests showed positive results in only 46.7% and 50.0%, respectively, of food-allergic patients. We conclude, first, that through intragastric provocation under endoscopic control (IPEC), food-allergic reactions on the mucosa of the GI tract can be verified and, second, that the liberation of tissue histamine seems to play an important role in the establishment of food-allergic reactions on the mucosa.