The incidence of Theiler's disease is rather low, but the disease is one that is likely to appear in any horse practice and it is well that every practitioner should know about it so that he will not become confused when it appears. It is my purpose to present a typical case and to explain the circumstances under which the disease can occur so that every practitioner will be alert to the possibilities of it in cases of nervous, icteric syndromes in horses. The case in point concerns a 12 year old bay saddle mare.