DURING the winter and spring of 1946 a severe epidemic of measles took place in Philadelphia. As is usual in epidemics of this disease, there was an increase in the number of complications, one of which was encephalitis. Thirteen thousand and thirty-seven cases of measles were reported to the Health Department. Fifteen deaths occurred in this entire group. Among these 13,037 cases, as far as can be ascertained, were 14 cases of measles encephalitis. The ratio of measles encephalitis was 1 to 1,000 reported cases of measles. It was our impression from the clinical and necropsy observations that edema of the brain was responsible in a great measure for the symptoms and fatal outcome in several of the patients. Dehydration therapy was instituted in some of our patients in an effort to correct cerebral edema and to study its effects on the symptoms and prognosis of the disease. Whenever it