The cerebrospinal fluid of a patient with Devic's syndrome contained antiglial fibrillary acidic protein antibody. The serum level of antibody was less than that in cerebrospinal fluid, and the antibody was probably synthesized within the central nervous system. Similar antibody was not found in another patient with Devic's syndrome or in patients with multiple sclerosis. The role of the antibody in the patient's illness is uncertain, but is one of the few instances in which antibody against a specific brain antigen has been described in human demyelinating disease.