In the treatment of acute respiratory failure, pulmonary barotrauma (subcutaneous emphysema, pneumothorax, and pneumomediastinum) developed in ten patients (10%) receiving IPPV without PEEP and in seven patients (11%) receiving IPPV with PEEP. Pre-existing chronic obstructive pulmonary disease seemed to predispose to the development of pulmonary barotrauma. Necropsy findings revealed the presence of pneumonitis and pulmonary edema in the majority of patients, in addition to pulmonary emphysema in some. Available information fails to demonstrate a correlation between the incidence of pulmonary barotrauma and the magnitude of air-way pressure required for adequate mechanical ventilation. The addition of PEEP to IPPV during therapy for acute respiratory failure does not in-crease the incidence of pulmonary barotrauma.