The discussion to be entered into in this report will be limited to an experience dealing with epidemic influenza as it was met with in the emergency Military Hospital in Pittsburgh. We shall largely confine our attention to the observations which came directly under our supervision, and in as much as this investigation was continued during the epidemic as it swept over this district, the intensive study was limited to a time period of about five weeks. During this period much material was collected, which since then, has taken us a considerable time to analyze. We have thought it more valuable to restrict our discussion to this material in that it illustrates the pathological lesions as they occurred during the acute stage of the disease. We have not entered upon a discussion of the sequelæ or the chronic lesions which are not uncommonly found following in the wake of an acute epidemic nor do we deal with the lesions arising in cases of sporadic influenza, such as are always with us. As is so well illustrated in the literature, there is probably no disease which has so many late complications and sequelæ as influenza. The investigations upon the protean lesions have been fully reported in numerous papers during the intervals between epidemics. A comprehensive bibliography upon influenza will be found at the end of the extensive report by Leichtenstern (1905). There is very much less accurate information available upon the actual lesions present during the acute disease when present in epidemic or pandemic form, than upon the many clinical complications in various systems and organs. In fact, our knowledge of the pathology of influenza lies more largely in the field of associated lesions such as the late events in the bronchi, the sinuses of the head, abscesses, meningitis and other conditions, rather to be viewed as complications than as portions of the disease. There are relatively few thorough pathological analyses of the influenza lesions as they are found in the acute epidemic disease.