Plasma levels of d-xylose one hr after a standard 5 g oral dose were measured in 82 children suspected of having celiac disease. Duodenal mucosa was obtained by intestinal biopsy in all. Among patients with normal mucosa, 95% had xylose levels above 20 mg/dl; 100% of patients with a flat mucosa had levels under 20 mg/dl. Results were independent of patients age. We conclude that this is a reliable test to screen patients suspected of having celiac disease, prior to intestinal biopsy.