ALTHOUGH sporadic reports of measles encephalitis are found in the literature prior to the year 1923, it is only in the last two decades that encephalitis as a complication of measles has seemed to occur in epidemic proportions. It is difficult to determine the exact incidence of this condition, as not all cases of measles are reported and many cases of mild encephalitis may be missed. Stimson1 stated the opinion that this complication usually develops in less than 1 per thousand cases. Table 12 discloses the incidence in previously reported epidemics of measles and also in our series. There were 10 additional cases at another contagious disease hospital which are not included. From January through June of 1946, 307 patients were admitted to the Cook County Contagious Hospital with the diagnosis of measles. Table 2 divides these cases into three separate age groups, showing the number of cases