A 7-year-old unimmunized girl with pertussis presented with respiratory failure and electroencephalographic evidence of an encephalopathy. The cerebrospinal fluid (CSF)/serum ratio of antibodies to pertussis toxin and filamentous hemagglutinin were 11- and ninefold higher than the CSF/serum ratio of total immunoglobulin G. The CSF/serum ratio of albumin was normal. These findings indicate production of antibodies in the central nervous system to Bordetella pertussis antigens and imply, therefore, that the pertussis encephalopathy in this girl was associated with the entry of pertussis antigens into the central nervous system.