Dawson's encephalitis is a progressive, fatal, degenerative disease of the central nervous system due to the measles virus. The clinical course of progressive intellectual and personality changes, myoclonic seizures, and increasing neurologic deficit may progress rapidly over several weeks. Early diagnosis depends on clinical awareness and may be confirmed by electroencephalography, spinal fluid gamma globulin levels, and measles antibody titers. Early diagnosis is imperative if therapy is to result in a functional child. The preliminary results of treatment with pyran copolymer and 5-bromo-2-deoxyuridine are presented. © 1969 The C. V. Mosby Company.