The permeability of the intestine was studied in 39 children (1 month to 3 years of age) with diarrhoea and in 28 children (6 months to 15 years of age) undergoing duodenal biopsy. Permeability was measured by differential absorption from an isotonic oral load containing 3.5 g lactulose, 0.5 g L-rhamnose, 0.5 g D-xylose, and 5 g lactose. Urinary sugar excretion was determined by quantitative thin-layer chromatography. Children with acute gastroenteritis had a greatly increased permeability, with a mean lactulose/L-rhamnose excretion ratio of 0.43 +/- 0.31 (normal less than 0.07). Children retested 3-16 weeks after complete recovery of their gastroenteritis had normal permeability (0.045 +/- 0.018). Children with chronic diarrhoea also had an increased permeability (0.12 +/- 0.074), but significantly less than the acute gastroenteritis group (p less than 0.01). Abnormal proximal small bowel morphology was associated with increased permeability, and a strong correlation between crypt depth and permeability was observed (r = 0.66, p less than 0.001). Abnormal intestinal permeability was associated with diarrhoeal disease and with mucosal damage. It appears to be a reliable and useful index of mucosal integrity.