The intestinal permeability test is a noninvasive method which, when done during a food provocation procedure, can detect the deleterious effect of food on the intestinal mucosa in allergic children. We report on a 1-month-old breast-fed boy with a history of regurgitation, diarrhea, difficult feeding, and malaise suggesting food allergy. Intestinal permeability tests were done with the mother's milk and showed breast-milk-induced alterations of intestinal permeability. No improvement occurred in the child's clinical symptoms or in the results of the intestinal permeability test when the mother withdrew dairy products from her diet. Disappearance of the child's symptoms and normalization of intestinal permeability during provocation with the milk were obtained after elimination of egg and pork from the mother's diet. This observation suggests that dietary proteins different from cow's milk antigens may be transferred to breast milk and induce adverse reactions in hypersensitive infants.