Endotracheal tube resistance may increase upper airway resistance and the work of breathing beyond the normal range in adults of children who are chronically intubated and receiving intermittent mandatory ventilation. The purpose of this study was to measure the resistance of infant endotracheal tubes (IETTs) over the range of normal infant respiratory flows while systematically changing length, diameter, and carrier gas density. Resistance was measured in IETTs of 2.5, 3.0, and 3.5 mm inner diameter at flows increasing from 10--150 ml/sec, using both air and 80% helium- 20% oxygen (He-O2) as the carrier gases. Tube length was progressively shortened from full length to 4.8 cm in 2-cm increments, and resistance to both air and He-O2 was measured at all lengths. Reynolds' number calculations and the relationship of resistance to flow showed the flow regime in IETTs to be transitional in nature. Increasing tube diameter, decreasing length, or decreasing gas density led to large decreases in IETT flow resistance. Over the range of normal infant flows, IETT resistance is equal to or higher than that of the normal upper aiaway. IETT resistance might increase the work of breathing in some infants to the point of respiratory failure, and such resistance may be lowered by systematic changes in IETT diameter, length, or carrier gas density.