Passive permeability of small intestine to lactulose and mannitol was studied in children suspected of having intestinal disease, using a hypertonic differential sugar absorption test. Children with coeliac disease and cows' milk intolerance were shown to have an elevated urinary lactulose/mannitol recovery ratio when compared with controls, children with normal jejunal biopsy, or children in whom alternative explanations for failure to thrive had been demonstrated. The abnormality in permeability ratio principally reflects a reduced urinary recovery of mannitol following oral administration. This study demonstrates that a hypertonic sugar absorption test is well tolerated even by young children, is practicable in routine clinical pediatric practice, and is capable of demonstrating abnormal intestinal function in children with small intestinal disease.