The majority of the fatalities occurred in the acute stage of the disease, the anatomical aspects of which have been elaborated more or less completely.[1] Other cases survived for a longer period and in these, anatomical changes existed, which, as will appear later, were prognosticated from the acute lesions. Moreover, these findings suggested that certain progressive anatomical changes occur even when the disease runs a much less severe clinical course; for example, in cases where respiratory symptoms persist for a long period before they are brought to a fatal conclusion. History, too, suggests such a chain of events; namely, in the record of the delayed crop of respiratory disorders that followed the harvest of the epidemic of ’90. Previous studies of experimental pneumonia in normal and aplastic animals by one of the authors (160) give a background for the interpretation of the histology of some phases of this disease, but more important are the studies of the respiratory inflammatory processes initiated by the inhalation of toxic gases. This subject, introduced into human Pathology with the use of poisonous gases in modern warfare, necessitated elaborate investigations which have just been concluded (159). The pathology produced by the inhalation of these poisonous vapors is analogous to that found in influenzal pneumonia. This is said with a full comprehension of the criticism that may follow such a statement, and with the knowledge that a similar analogy has been drawn between influenzal and plague pneumonia (Symmers, 141). It is, however, a criticism that is welcomed and which will be met in the body of the paper. For the reasons just cited, it has seemed desirable to contribute to the Pathology of various phases of influenzal pneumonia and to attempt to correlate this with other types of acute respiratory inflammation, in the hope that the prognostications which suggest themselves may be of aid in the prophylaxis and possibly in the treatment of the more insidious and progressive pulmonary changes that may follow this disease. A large part of the text is devoted to a description of the gross and more minute pathology of the respiratory tract associated with influenza and its complications, both in the acute and in the subacute or chronic stages. Incidental lesions of less importance in other portions of the body that have occurred in these cases are presented briefly, and emphasis is placed upon a number of special subjects. The order of discussion will be as follows:— I. The Pathology of the Respiratory Tract in Influenza. (A) Lesions of the Trachea and Bronchi. (B) Lesions of the Lung. (1) Acute Diffuse Fulminating Type. (2) Localization and Necrotization of the Pneumonic Process. (3) Organization of the Bronchiolar and Pneumonic Processes. II. Influence of the Respiratory Complications of Influenza upon Tuberculosis of the Lung. III. Extrarespiratory Lesions in Influenza.