Humoral immune responses to Chlamydia pneumoniae (C. pneumoniae) were studied in paired sera and cerebrospinal fluid (CSF) of patients with definite multiple sclerosis (MS) and other inflammatory and non-inflammatory neurological diseases. Seropositivity was not significantly different between these groups. However, C. pneumoniae-specific IgG titers were significantly higher in CSF of MS than in controls. Sixteen out of 52 seropositive MS patients (30.8%) showed intrathecal synthesis of C. pneumoniae-specific IgG but only one of 43 seropositive controls (2.3%). In MS, this was strongly associated with intrathecal synthesis of polyclonal IgG in 13/16 patients. However, these elevated C. pneumoniae antibody titers in CSF did not significantly correlate with disease duration, disease course, clinical or MRI disease activity, disability or presence of oligoclonal IgG in MS.