A new analytical method gas chromatography combined with UV spectrophotometry was used to measure isoprene and acetone in expired breath collected from four different groups of children: 1) healthy newborn babies, 2) healthy preschool children, 3) healthy school children, and 4) diabetic children in different metabolic states. Both isoprene and acetone could readily be determined in one single analysis of a 250-mL air sample. Newborn babies during the first postnatal week had undetectable or very low levels of isoprene in their expired air irrespective of catabolic or anabolic state. Breath isoprene increased with age, and healthy school children had higher levels than did healthy preschool children. No significant differences in breath isoprene were found between healthy and diabetic children. Breath acetone was found to correlate with metabolic state both in newborn babies and in diabetic children. These findings illustrate the potential use of a new technique for breath analysis in children with metabolic disturbances.