Summary Attention has been drawn to the condition of localized bullous emphysema associated with pneumonia, which has been practically overlooked, even in the more recently published pediatric textbooks. This is certainly common enough to warrant a complete description in all volumes dealing with the diagnosis of diseases of the lung in infants and children. The term “cyst” or “cystic” should not be applied to this condition. In contrast, true congenital pulmonary air cyst is encountered very rarely. It should be emphasized that all our cases have been associated with pneumonia and frequently with some degree of pleurisy, often quite localized in the affected region. There may be an accompanying empyema or, if rupture of the pleura occurs, pneumothorax. We have not encountered it in children with pulmonary tuberculosis without pneumonia; nor have we observed any such instance in roentgenograms of normal children's chests. The characteristic roentgenologic appearance of localized bullous emphysema consists of one or more abnormal areas of diminished density, surrounded by thin, dense, smooth margins. These are spheroid or ovoid in shape, and sometimes loculated. They are easily seen when the surrounding lung is comparatively clear; on the other hand, they may be merely suspected at first when infiltration is present in the contiguous lung, but they become more definite as this clears in subsequent examinations. Their location is usually subpleural or intrapleural. They may contain some fluid, but as a rule only a small amount. The size varies from very small, when they may be hardly distinguishable, to voluminous balloonlike spaces, which may occupy a large portion of one lung. Variations in size occur from time to time until they eventually disappear. Usually this takes place in a few weeks, but occasionally some months may elapse before the chest returns to normal. This condition is not accompained by any abnormal signs or symptoms, unless enormous inflation results or the pleura ruptures with the formation of pneumothorax, giving rise to evidence of increased intrathoracic pressure. The differential diagnosis between bullous emphysema and lung abscess, localized pneumothorax, congenital pulmonary air cyst, and diaphragmatic hernia is not difficult, if the course of the case can be followed for a short time. No treatment is necessary unless complicating conditions ensue.