Experimental and clinical study of postoperative apneumatosis and postoperative pneumonia, carried on for several years, has convinced me that these two complications represent two phases of the same morbid condition. The differences between them are differences only in degree, and they depend on the type and virulence, of their most frequent causative organism, the pneumococcus. Otherwise they have the same etiology and the same mechanism of production; they present a similar clinical evolution and they show similar pathologic changes. Furthermore, they are often superimposed in the same case and give rise to similar complications. The experimental work on atelectasis and lobar pneumococcic pneumonia carried on with the collaboration of Dr. Birnbaum,1in the Laboratory of Surgical Research at Cornell Medical College, led me to the following conclusions: 1. There is only one determining cause of atelectasis and that is bronchial obstruction. 2. This occlusion must be complete. 3. Neither