Controversy exists regarding the routine use of positive end-expiratory pressure (PEEP) in mechanically ventilated patients. We hypothesized that nonhypoxic patients receiving 5-cm H2 O PEEP would have improved shunt and PaO2 /FIO2 ratios (P/F), without an increased dead space to tidal volume ratio (VD/VT) versus patients receiving no PEEP. Forty-four trauma patients were randomized to receive 5-cm H2 O PEEP (PEEP) or 0-cm H2 O PEEP (ZEEP). Shunt VD/VT and P/F were measured at 0, 12, 24, 36, and 48 hours after intubation and after extubation. PEEP and ZEEP comparisons used Student's t test and the General Linear Models procedure. Shunt was significantly increased at t = 0 and at extubation in the PEEP group. At extubation, the PEEP group demonstrated significantly higher VD/VT and poorer P/F ratios. After correction for baseline values, no statistically significant differences were noted in spite of a trend toward worsening pulmonary function in all measured parameters. These results suggest that routine use of 5-cm H2 O PEEP in mechanically ventilated trauma patients is not necessary.