Intrathecal synthesis of IgM and IgG, oligoclonal immunoglobulin bands, and the levels of fibronectin, soluble interleukin 2 receptor, interleukin 6, and tumor necrosis factor alpha were investigated with the use of enzyme-linked immunosorbent assay in 46 paired cerebrospinal fluid and serum samples from 32 patients with meningopolyradiculoneuritis due to Borrelia burgdorferi (Lyme borreliosis stage 2). Cerebrospinal fluid and serum interleukin 6, although not specific for neuroborreliosis, were good indicators of disease activity, while the serum soluble interleukin 2 receptor level was only mildly elevated. Tumor necrosis factor alpha was never detected in cerebrospinal fluid or serum specimens, and cerebrospinal fluid IgM, IgM index, and cerebrospinal fluid IgM/cerebrospinal fluid IgG ratios were significantly higher than in all other neuroimmunologic disorders evaluated and may be valuable diagnostic indicators for neuroborreliosis. The estimation of intrathecally synthesized IgG and IgM fractions for the differential diagnosis of neuroimmunologic disorders did not add to IgG and IgM index calculations.