Borrelia burgdorferi, the causative agent of Lyme borreliosis may affect both the central and the peripheral nervous system causing neuroborreliosis. In Europe neuroborreliosis is the most frequent manifestation of late Lyme borreliosis. In this study we investigated characteristics of intrathecal antibody synthesis in patients with neuroborreliosis. Our goal was also to reveal to what extend anti - B.burgdorferi antibody detection in serum correlates to anti - B.burgdorferi antibody detection in CSF. We examined 266 patients with clinically suspected neuroborreliosis, and found serologic evidence of B.burgdorferi infection in 94 (35 percent). By calculation of IgG and IgM specific antibody indices, we detected intrathecal antibody production in 49 (18 percent) of patients. IgM antibody response dominated in the cerebrospinal fluid (CSF) samples and IgG antibody response was prevalent in the serum samples. Manifestations of neuroborreliosis included cranial nerve affection (n=12), radiculitis (n=10), meningoradiculitis (n=11), encephalopathy (n=7), radiculomyelitis (n=6), and encephalitis (n=3). No patients with either MS-like syndrome (n=15) or motor neurone disease (n=8) had intrathecal B.burgdorferi-specific antibody production despite positive serum reaction for the specific antibodies. Most of the patients with documented intrathecal antibody synthesis (42/49-86 percent) improved significantly after etiologic treatment. In the rest of patients, mainly those with long-terming neurological disorders, the improvement was partial and temporal. We concluded, that detection of antibodies against B.burgdorferi in serum should always be completed with detection of intrathecal antibody synthesis in CSF in order to confirm clinical diagnosis of neuroborreliosis.