Proximal airways are compliant structures at early gestational ages and may be susceptible to pressure-induced deformation following prolonged ventilatory support. Sixteen neonates (mean +/- SD gestational age, 27.0 +/- 0.6 weeks; mean +/- SD birth weight, 847 +/- 68 g) were studied to assess tracheal volume deformation. The neonates received ventilatory support for a mean +/- SD duration of 25.4 +/- 4.9 days. During this period the maximum peak inspiratory pressures ranged from 15 to 25 cm H2O, and respirations ranged from 20/min to 60/min. These neonates were studied at seven days postextubation and were individually matched for body weight with 16 nonventilated neonates. The width of the tracheal air column was measured at the lower border of the first thoracic (T-1) and third thoracic (T-3) vertebrae. The average tracheal width (average of T-1 and T-3) was significantly (38%) wider in the ventilated group, and the mean +/- SD tracheal width values were 3.79 +/- 0.29 mm, as compared with the control values of 2.74 +/- 0.31 mm. Based on these data it was estimated that the tracheal volume was 91% greater in the ventilated group. These observations demonstrate tracheal volume deformation and acquired tracheomegaly in neonates who have received mechanical ventilatory support. In addition to increased dead space ventilation, these findings also indicate underlying mechanical deformation of the tracheal wall.