We used a mixture of low molecular weight polymers of polyethylene glycol (PEG) to study intestinal permeability in children with chronic diarrhea. After an oral load, urinary recovery of the longer chain, higher molecular weight polymers was significantly decreased compared with that in healthy adults and infants without disease of the gastrointestinal mucosa. A correlation was observed between clinical severity of disease, histopathologic condition of the small bowel mucosa, and PEG scores. These results indicate that intestinal permeability in children can be studied by noninvasive means and that an assessment of mucosal integrity can be obtained.