The differential passive permeation of polymers of different lengths across the intestinal mucosa has been proposed as a probe to test mucosal integrity under a variety of physiologic and pathologic conditions. Polyethylene glycol (PEG) 400 contains a series of polymers whose pattern of urinary recovery after oral administration has been used to characterize intestinal mucosal function. To extend this method to the low levels of PEG polymers found in the urine of children with diarrhea, we have introduced three methodologic innovations. These alterations involve (1) formulation of a balanced PEG polymer mixture, (2) improved isolation, derivatization, and gas chromatography techniques, and (3) a new quantification of the pattern of PEG urinary recoveries. Urinary recovery of orally administered PEG was assessed in four normal adults, six hospitalized infants without gastrointestinal complains, two infants with prolonged diarrhea and carbohydrate malabsorption, and two children with cystic fibrosis. A parameter characterizing the urinary recovery of PEG, N1/2, which is the theoretical number of subunits in the polymer whose recovery is reduced to 50% of the value of the polymer whose recovery is maximal, gave stable, reproducible, and consistent results in normal adults and infants.