We reviewed serial chest roentgenograms of 82 patients treated with positive end-expiratory pressure (PEEP) therapy for morbid pulmonary failure. No change was identified in the appearance of the chest roentgenograms in 49 patients. Thirty-three patients showed pulmonary hyperinflation, varying degrees of apparent clearing of alveolar or interstitial disease, and resorption of interstitial and pleural fluid. Lung hyperinflation and interstitial gas generally preceded pneumomediastinum and pneumothorax. These pulmonary complications were identified roentgenographically, often before they became clinically apparent. No consistent correlation was observed between these roentgenographic changes and the ventilatory pressures employed. The radiologist should be informed that a patient is receiving PEEP therapy, as the earliest signs of alveolar rupture--interstitial emphysema--are frequently subtle.