Although numerous reports have described the improvement in PAO2 in patients in the prone position, the underlying mechanism has yet to be determined. Some authors have suggested this phenomenon may be related to an increase in functional residual capacity (FRC); however, no previous studies have described positional changes in FRC in children with severe lung disease or in those under neuromuscular blockade. We measured arterial blood gases, FRC, Rrs, and Crs in supine and prone positions in 30 patients under neuromuscular blockade with lung disorders including moderately severe restrictive (n = 10) and obstructive (n = 10) disease and control subjects without significant lung disease (n = 10). Prone positioning was not associated with a significant increase in FRC in the cohort of 30 patients, nor in any of the subgroups. Although individual patients demonstrated large improvements in oxygenation, a statistically significant (but clinically insignificant) increase in AaPO2 ratio was observed only in the subgroup of patients with obstructive disease (0.35+/-0.03 to 0.38+/-0.04, p = 0.027). There was no correlation between changes in FRC and changes in AaPO2 (r = 0.225, p = 0.23). A significant improvement in Rrs occurred in the prone position compared to supine in patients with obstructive lung disease, decreasing from 0.264+/-0.024 to 0.216+/-0.021 cm H2O/ml/s, p = 0.009. No significant changes in Crs were seen in the prone position. We conclude that prone positioning has no effect on FRC and in this series of 30 patients significantly improved oxygenation only in patients with obstructive airway disease. A significant decrease in Rrs in patients with obstructive lung disease was also observed.