Clinical data of 19 patients with meningopolyneuritis Garin-Bujadoux-Bannwarth (MPN-GBB), treated with 2 X 10 million units intravenous sodium penicillin for 10 days, were evaluated at the beginning of therapy, 3 weeks thereafter, and 6 months after onset of the neurological disease. Cerebrospinal fluid (CSF) was analysed in 14 patients at the onset of therapy and 3 1/2 weeks thereafter. At the same interval antibodies against B. burgdorferi were measured by enzyme-linked immunosorbent assay (ELISA) in the CSF and sera of 12 patients. Clinical data and all CSF results, with exception of specific antibody titers, were compared with those of patients who had suffered from MPN-GBB between 1979 and 1983, and who had not received antibiotic or corticosteroid therapy. Comparing the clinical data of all treated patients with those of all non-treated controls, no significant difference could be observed. A significant improvement could however be detected in those patients who had their treatment begun 5 weeks within onset of the neurological disease. Changes in CSF 3 1/2 weeks after onset of treatment showed slight differences when compared with controls.