In an 18-month period, we treated 561 patients with mechanical ventilation. Fifty-four (10 percent) of these patients had acute respiratory failure, requiring treatment with positive end-expiratory pressure (PEEP) in excess of 20 mm Hg (range, 20 to 40 mm Hg). All patients were allowed to breathe spontaneously between volume-limited mechanical breaths delivered at a rate sufficient to maintain an arterial pH greater than or equal to 7.35. PEEP was applied until calculated pulmonary venous admixture was minimized. Forty-three (80 percent) of these 54 patients were alive and asymptomatic three months after dischage from the hospital, and tests of pulmonary function were performed on ten patients within one year after hospitalization. Abnormalities in pulmonary function appeared to be reversible, and pulmonary function gradually approached normal within one year. It appears that neither acute respiratory failure nor exposure to high airway pressures caused significant permanent pulmonary damage in the ten patients studied.