The antibody response against a spirochetal strain isolated from Swedish Ixodes ricinus ticks was determined by enzyme-linked immunosorbent assay (ELISA) and indirect immunofluorescence assay of cerebrospinal fluid (CSF) and serum specimens from 45 patients with chronic meningitis. Samples of CSF, serum, or both from patients with various infections of the central nervous system, multiple sclerosis, syphilis, or infectious mononucleosis and from healthy individuals served as control samples. Probable spirochetal etiology could be demonstrated for 41 of 45 (91%) patients with clinical symptoms of chronic meningitis. Approximately 25% of the patients had significantly elevated titers of antibody to the spirochete in CSF but not in serum. The highest diagnostic sensitivity, 91%, was demonstrated by measurement of CSF antibodies and calculation of a spirochetal CSF titer index, which is the ratio of (ELISA titer in CSF/ELISA titer in serum) to (albumin in CSF/albumin in serum) and which also considers the degree of blood-CSF barrier damage. The highest specificity, 98%, was obtained by calculation of a CSF titer index. Patients with short duration of disease were especially prone to be antibody negative in serum but positive in CSF. Significant rise in serum antibody titers was seldom demonstrated in patients treated with antibiotics. It is concluded that measurement of CSF antibodies, especially by ELISA, is a highly sensitive and specific method for the immunological diagnosis of spirochetal meningitis.