Data from serial determinations of fecal volume, characteristics, pH, and reducing substances, and postprandial breath hydrogen (H2) concentrations in severely malnourished children receiving milk-based recovery diets were used to evaluate the quality of the correspondence among diagnostic indices for carbohydrate malabsorption, with specific emphasis on the comparison of breath H2 excretion and fecal reducing substances. Only postprandial breath H2 results for subjects with a proven capacity to mount an H2 response to the nonabsorbable disaccharide, lactulose, were included. There was a poor correlation between the diagnostic indication provided by the breath test and fecal reducing sugars. Biological considerations about the metabolism of unabsorbed carbohydrates in relation to colonic transit time and fecal flora may explain the poor degree of correlation. The two indices should not be considered interchangeable, but should be used selectively in accordance with the clinical situation.