Changes in intestinal permeability and lactose hydrolysis have been investigated in three adults and fifteen infants with acute rotaviral gastroenteritis by differential sugar absorption. The method involves chromatographic measurement of urinary lactose, lactulose and L-rhamnose excretion following combined ingestion in an iso-osmolar test solution. All patients had abnormal intestinal permeability indicated by raised urine lactulose/L-rhamnose excretion, ratio of percentages recovered in 5 h, of 0.462 (0.100-1.227) mean and range, compared with 0.027 (0.008-0.052) for healthy controls (P less than 0.001). Ten patients also had urinary lactose/lactulose excretion ratios raised above the normal range (0.014-0.41, mean 0.258) during their acute illness, indicating impaired intestinal lactose hydrolysis. Both indices had become normal 4 weeks after the acute illness, serial investigation of five patients showing that improvement was complete much earlier. Except for the short duration these changes are similar to those associated with villous atrophy in coeliac disease. The test procedure was verified with respect to intestinal lactose hydrolysis by demonstrating a linear relationship between lactose/lactulose excretion and log jejunal mucosal lactase activity by in vitro assay (R2 = 0.95) in a further group of subjects. Differential lactose/lactulose/L-rhamnose absorption provides a non-invasive and sensitive index of small intestinal integrity of value for the interpretation of prolonged or otherwise complicated enteritis and the distinction of primary secondary intestinal lactase deficiency.