In adults, anatomic dead space is 2.2 ml/kg. Because of the relatively large head size of infants and children, we hypothesized that extrathoracic and, therefore, total dead space would be relatively larger in pediatric subjects. Extrathoracic dead space was measured by a "water displacement" technique in 40 patients aged 7 days to 14.2 yr who were intubated with cuffed endotracheal tubes. Intrathoracic dead space was measured by continuous analysis of end-tidal and mixed-expired PCO2 and minute ventilation in 10 patients, aged 18 days to 14.7 yr. Extrathoracic dead space per kilogram decreased exponentially with increasing age, ranging from 2.3 ml/kg in early infancy to 0.8 ml/kg in children older than 6 yr. Mean intrathoracic anatomic dead space was 1.03 ml/kg and was not related to age. The following relationship between total anatomic dead space (DStotal; in ml/kg) and age (in yr) is derived: DStotal = 3.28 - 0.56 [ln(1 + Age)], with r = 0.95 and P = 0.0001. Anatomic dead space is age dependent and is > 3 ml/kg in early infancy.