A large number of cases of an acute disease characterized by areas of demyelination (usually perivascular) with relative preservation of axis cylinders and infiltration with round and glial cells are now on record.1This disease or group of diseases has been called by various names, such as acute multiple sclerosis, acute disseminated encephalomyelitis and degenerative encephalopathy. In many instances, the condition has followed acute infectious diseases, especially vaccinia and measles. A similar disease, occurring spontaneously in dogs, was described by Perdrau and Pugh.2It was produced experimentally in dogs by Claude3by the injection of minimal doses of tetanus toxin, and by Ceni and Besta4by Aspergillus spores. We have succeeded in confirming Claude's work.5 The relation between this group of diseases and multiple sclerosis has been the subject of lively controversy ever since cases of acute demyelination were first reported. On the one hand