ABSTRACT—A newly identified spirochete,Borrelia burgdorferi, has recently been established as the causal agent of chronic meningoradiculitis (Bannwarth's syndrome, BS). An etiological diagnosis can be obtained by specific antibody determination. To detect intrathecally producedBorreliaantibodies, we examined paired serum and CSF samples from 10 BS patients and 41 controls. CSF/serum IgG specific antibody indices were calculated by relating the CSF/serum ratio of specific antibody activity to the CSF/serum ratio of total IgG and of albumin; 8/10 BS patients compared to 4/39 controls had elevated index values. In only one of the 4 positive controls could spirochetal infection be excluded. We conclude that determination of specific antibody indices is a reliable and sensitive diagnostic test for nervous systemB. burgdorferiinfectio