In syphilitic patients with or without CNS involvement the correlation of Treponema-specific IgG titre per milligram total IgG in CSF and serum (ITPA index) is a dependable source of information on the synthesis of treponemal IgG antibodies in the CNS. This index also provides a more reliable definition of asymptomatic neurosyphilis. Further, a discrimination between Treponema-specific and Treponema-non-specific IgG synthesis in the CNS is possible. Of 261 patients with clinical symptoms of neurosyphilis, 82% had a local production of treponemal IgG antibodies as shown by an elevated ITPA index. In patients with neurosyphilis and intrathecal synthesis of Treponema-specific IgG antibodies, 94% had oligoclonal IgG in the CSF. Comparison of different CSF protein alteration groups in untreated and treated neurosyphilitic patients showed that early diagnosis (and early treatment) led to improvement of the impairment of the blood-CSF barrier and reduction of the immune reaction in the CNS. However, synthesis of treponemal IgG antibodies in the CNS could persist as a 'scar syndrome' even after adequately cured infection.