Chronic lung disease in the form of stage III or IV bronchopulmonary dysplasia (BPD) was found to occur among nine of 188 infants receiving intermittent positive pressure ventilation for respiratory insufficiency. None occurred in infants ventilated with a mask alone. The duration of endotracheal intubation was not significantly longer in patients developing BPD; however, the duration of exposure to inspired oxygen > 60% was significantly longer in that group (mean, 141 hours.). A previously undescribed form of chronic lung disease was found to occur among nine of 150 infants ventilated because of underlying idiopathic respiratory distress syndrome. A radiographic appearance of diffuse haziness with loss of identifiable lung markings occurred at 5 to 15 days of age and was not associated with clinical signs or symptoms or increased oxygen needs. These changes disappeared in one to five days.