Positive and expiratory pressure (PEEP) has been used for several years in the treatment of acute respiratory failure. Dramatic improvement in the chest radiograph may occur with institution of PEEP; the degree of change parallels the levels of positive and expiratory pressure used. Conversely, weaning from mechanical ventilation may be associated with deterioration in the radiographic picture despite improvement in arterial blood gases and lung compliance. Serial chest films of representative patients with adult respiratory distress syndrome are reviewed and changes in the radiographic pattern are correlated with the amount of PEEP. The efficacy of PEEP in reducing intrapulmonary shunting and improving arterial oxygenation is related to increasing functional residual capacity, with improvement of diffuse atelectasis and associated shift of pulmonary water from the alveoli to the interstitial space and pulmonary capillaries. The changing radiologic manifestations reflect these physiologic phenomena. Pulmonary barotrauma is a frequent complication of PEEP therapy. Pneumothorax, pneumomediastinum, and interstitial emphysema may lead to rapid deterioration of a patient maintained on mechanical ventilation with an already compromised respiratory status.