Positive end-expiratory pressure (PEEP) may improve pulmonary mechanics, work of breathing, and gas exchange in some patients with respiratory failure. These beneficial effects do not occur consistently, however, and side effects, such as gas trapping due to expiratory flow limitation, may be exacerbated. We determined the effects of PEEP (0, 3, 6, and 9 cm H2O applied in random order) on the expiratory airway resistance and static compliance of nine infants mechanically ventilated for acute bronchiolitis. We also noted the presence of inadvertent PEEP (PEEPi) to determine its influence on the response to applied PEEP. Applied PEEP at any level failed to consistently improve passive expiratory airway resistance or increase compliance from baseline (PEEP = 0 cm H2O, resistance = 92 ± 32 cm H2O/L/s; compliance = 0.71 ± 0.19 ml/cm H2O/kg). Increases in end- expiratory lung volumes ranged from 18 to 40% of the tidal volume at maximal PEEP. Although all infants had PEEPi (5 ± 2 cm H2O), PEEPi had no influence on the response of mechanics to applied PEEP other than that peak inspiratory pressures increased when PEEP > PEEPi. We conclude that the routine use of PEEP in infants with bronchiolitis does not consistently improve passive expiratory pulmonary mechanics and may increase the risk of barotrauma from gas trapping.