Thirty-six children with suspected gastrointestinal hypersensitivity to cow's milk protein were investigated before and after challenge with cow's milk protein by one or more of four tests of gut function: the appearance of small bowel mucosa, mucosal disaccharidase levels, a 1-h blood-xylose test, and a 50 g-lactose breath-hydrogen test. These tests were not always abnormal in children who had definite adverse reactions to milk. Conversely changes were seen in some with negative milk challenges. Although small bowel biopsy, assessment of disaccharidase activity and perhaps the breath-hydrogen test have an important place in the pre-challenge assessment to exclude other causes of gastrointestinal symptoms, these gut function tests and the 1-h xylose test done following milk provocation do not appear to have any advantage over careful clinical observation.