This case report involves an aged, grade gelding used as a children's hunter. The horse was presented to the clinic in a critical state and died in a coma the same day. The history and examination of the horse lead to a diagnosis of Theiler's disease, which was based upon previous inoculation with homolgous serum, intense icterus, cerebral depression and excitement. Laboratory findings and necropsy confirmed the diagnosis with an SGOT of fifteen times normal, an icterus index of 200; and diffuse and massive liver necrosis. Supportive treatment was undertaken, and a proposed schedule of possible therapeutic measures was discussed in dealing with the physiological changes brought about by the acute liver destruction. A review of the literature begins with observations of Sir Arnold Theiler who first described the syndrome in 1914 while working with African Horsesickness at Onderstepoort, Union of South Africa. Theiler's disease first made its appearance in the United States during the 1930's at which time it followed epizootics of viral encephalitis in horses. These outbreaks are discussed as well as those appearing in other parts of the world. The etiological agent discussed, although speculatively, is a virus, based on the epidemiological nature of Theiler's disease in the past, and the resemblence of this syndrome in horses to viral, infectious and serum hepatitis in man. Finally, control of Theiler's disease in possible future outbreaks is discussed.