In young infants the clinical and investigative features of coeliac disease (CD) may be mimicked by other conditions such as cow's milk intolerance or secondary disaccharidase deficiency. It is therefore especially important to confirm a diagnosis of CD by later gluten challenge in such infants. Sixteen children in whom the diagnosis of CD had been made before the age of 12 months had an oral gluten challenge, after being treated with a gluten-free diet for periods of one month to 5 years. In 15 we showed intestinal xylose malabsorption by the one-hour blood xylose level within 1-28 days of starting ingestion of gluten. One child, with a persistently normal one-hour blood xylose test after gluten challenge for 3 months, had normal absorption and normal jejunal histology after 18 months on a gluten-containing diet; she is considered not to have CD. The one-hour blood xylose test before and after gluten challenge can help to confirm the diagnosis in coeliac patients diagnosed in infancy.