Cerebrospinal fluid (CSF) and serum from 45 patients with lymphocytic meningoradiculitis were examined by isoelectric focusing combined with immunoblotting to detect Borrelia burgdorferi-specific oligoclonal immunoglobulin G (IgG) bands. In pretreatment samples, 35 patients (78%) showed B. burgdorferi-specific oligoclonal IgG in CSF indicative of intrathecal antibody production. At 2, 3-6, and 6 weeks after onset, respectively, such bands were present in 5 (42%) of 12, 21 (88%) of 24, and in all of 9 patients (100%). Up to 1 year after therapy, specific oligoclonal bands in CSF tended to remain unchanged despite clinical recovery. B. burgdorferi-specific oligoclonal bands in serum were found in 7 patients. These bands had identical migration patterns as in CSF, but were fewer in number and in some patients showed a temporal evolution different from their CSF counterpart. Not all oligoclonal IgG in CSF reacted with B. burgdorferi. The 41-kDa flagellar antigen was shown to be a major antigen in the intrathecal immune response. The demonstration of B. burgdorferi-specific oligoclonal IgG in CSF is a sensitive and reliable indicator of Lyme neuroborreliosis.