IN RECENT years instances of acute disseminated encephalitis are being recognized with progressively increasing frequency. This increase probably represents a true rise in the incidence of such disease, not merely an apparent rise. For many years the term "encephalitis" was used to refer specifically to viral or bacterial diseases or to conditions suspected of having such an origin. Recently, it is becoming increasingly apparent that many, if not the majority of, encephalitides are noninfectious in origin and that they may well represent the acute stages of conditions previously considered degenerative. For the sake of clarity, it is well to consider three types of disseminated encephalitis, namely, the infectious, the definitely noninfectious and the diffuse demyelinating diseases of questionable origin. Many authors have attempted to subclassify the last type, the so-called acute demyelinating diseases, but this endeavor is made difficult by the frequent coexistence of lesions in the gray and the