PEEP has been advocated for use in patients with the acute respiratory distress syndrome characterized by a marked reduction in FRC. It has not been advocated for patients with apparently normal lungs requiring prolonged mechanical ventilation. We have done inert gas washouts on five young patients comparing no PEEP to 10 cm H2O PEEP. Four of the five showed a significant improvement in PaO2, a decrease in AaDO2 and Qs/Qt, the fifth a slight improvement on PEEP. Arterial washouts were significantly faster on PEEP in all patients suggesting a decrease in intrapulmonary shunting. Airway washouts in the initial phase were faster off PEEP suggesting an increase in lung volume and FRC with PEEP. Later portions of the curve were faster on PEEP suggesting improved distribution of ventilation. These findings suggest that PEEP might be of significant value in treatment of patients requiring prolonged mechanical ventilation for reasons other than lung abnormalities.