In a previous publication, I1reported the results of studies in cases of postoperative pneumonia covering a period of eighteen months. These studies were undertaken to determine (1) the incidence of pneumonia following operations under general anesthesia, and (2) the etiologic factors concerned in its production. The conclusions reached were as follows: 1. Postoperative pneumonia occurs as a relatively frequent complication following operations on the gastrointestinal tract, the gallbladder and bile ducts, the appendix, and also following operation for hernias and exploratory laparotomies. It is of less frequent occurrence in operations on other parts of the body, the figures being less than 1 per cent. 2. The etiologic factors concerned are, the presence of infection in the respiratory tract at the time of the operation, hypostatic congestion in old and weakened persons, and irritation of the respiratory tract by a too liberal use of the anesthetic. 3. Aside from