Elevated breath hydrogen levels resulting from colonic bacterial fermentation of nonabsorbed carbohydrate identifies disaccharidase deficiency in adults. This principle was validated in children with sucrose intolerance by interval breath sampling using a nasal prong, thus permitting simple, noninvasive collection of expired air for H2 analysis by gas chromatography. Six symptomatic children (ages 6-11 yrs) with congenital sucrose intolerance (4 proven sucrase deficient by jejunal biopsy and 2 sibs) were compared with 6 normal controls (ages 5-8 yrs) and 2 children with biopsy-proven acquired sucrase deficiency now clinically well (ages 3 and 10). Expired air was collected before and at 30, 60, 90, 120 and 180 min after orally administered sucrose (2 gmAg; max 50 gm). Peak H2 excretion was significantly elevated in sucrose intolerant patients: 114±63 (meant S.D.) parts per million above baseline (Δppm), versus 2±2 Δppm (p<0.001) in controls and patients who had recovered. Best discrimination occurred at 90 minutes post-ingestion (range 28-203 vs. 0-3 Δppm). Breath H2 determination provides a sensitive, reliable means for detection of sucrose malabsorption. The nasal prong technique permits convenient application of this methodology to young children, and offers the opportunity to screen, diagnose, and evaluate therapy for malabsorption of specific carbohydrates in the pediatric age group.