A patient with proven borrelial infection of the central nervous system (CNS) and progressive weakness of the arms was treated with antibiotics. Although the initially elevated CXCL13 concentration in the cerebrospinal fluid decreased, indicating effective treatment of the infection, weakness progressed. Investigation revealed multiple nerve conduction blocks and the presence of GM1 antibodies, suggesting a multifocal motor neuropathy; the patient improved on treatment with intravenous immunoglobulins. This report of an autoimmune-mediated polyneuropathy in a patient with borrelial CNS infection indicates that such patients might respond to immunomodulatory therapy if antibiotic treatment is not effective. © 2007 Wiley Periodicals, Inc.