We studied 35 patients with chronic meningitis. The neurological abnormalities included aseptic meningitis, cranial neuropathy (mostly facial palsy), motor and sensory peripheral radiculoneuropathy, and myelitis. Neurological symptoms were sometimes preceded by erythema chronicum migrans or an insect bite and were often accompanied by fever, malaise, profound fatigue, and weight loss. The cerebrospinal fluid (CSF) abnormalities consisted of a predominantly mononuclear pleocytosis, an elevated CSF protein (mean 2.3 g/l), intrathecal synthesis of oligoclonal immunoglobulin G, and, in half of the patients, a fall in the CSF/blood glucose ratio. High antibody titers to the Lyme spirochete and the Swedish Ixodes ricinus spirochete were demonstrated by immunofluorescence in 26 of the 35 patients. By imprint immunofixation of electrofocused samples of serum and CSF, intrathecal production of oligoclonal Lyme-spirochete-specific IgG was demonstrated in one patient with chronic meningitis. Four sequential paired samples of serum and CSF from this patient showed local synthesis of spirochete-specific antibodies in CSF. The 35 patients improved or recovered, sometimes dramatically, during a two-week course of intravenous penicillin G.