Neurologic complications are common in Lyme borreliosis. In adults in particular, detection of intrathecally formed antibodies to B. burgdorferi is a decisive diagnostic criterion. As in neurosyphilis diagnosis, the specific antibody titer against B. burgdorferi in the CSF is compared with the serum titer by calculating the specific antibody indices, taking into account the disturbed blood/CSF barrier. Deriving from these specific indices, we adapt by dilution the serum IgG level to that in the CSF of the same day, thus allowing for passive diffusion of antibody through the blood/CSF barrier. Determination of the specific antibody titer by indirect immunofluorescence demonstrates intrathecal synthesis of specific antibodies against B. burgdorferi if the CSF titer is higher than in the diluted serum tested in parallel. The individual methods are presented in a group of patients.