The determination of serum levels of D-xylose and the urinary excretion of an orally administered mixture of low-molecular-weight polyethylene glycol were compared to assess their sensitivity to predict smallbowel mucosal damage. Eighteen infants with severe diarrhea and villus atrophy were observed. Results of the D-xylose and polyethylene glycol tests were compared with the villus-to-crypt ratios that were determined from biopsy specimens. The D-xylose test predicted accurately 12 (67%) of the 18 small-bowel biopsy results, while the polyethylene glycol test predicted 14 (78%). However, the McNemar test indicated that the two tests did not differ significantly in their ability to predict an abnormal small-bowel biopsy. We conclude that neither the D-xylose nor the polyethylene glycol test is a reliable indicator of small intestinal mucosal damage in infants with chronic diarrhea.