The aim of the study was evaluation of usefulness of cerebrospinal fluid (CSF) serotonin level examination in diagnostics of post-Lyme disease syndrome. The study was performed in 16 subjects. In all individuals CSF serotonin concentration was estimated on the 1st day of hospitalization. In patients with depressive and cognitive impairments, proved in neuropsychological tests, - group I--mean CSF serotonin concentration was 1,26 ng/ml, whereas in subjects without abnormalities in tests--group II--respectively--3,87 ng/ml. The difference of mean CSF serotonin levels was statistically significant (p<0,01). The obtained results indicate usefulness of this CSF parameter, besides neuropsychological tests, in objective evaluation of clinical state in patients with post-Lyme disease syndrome.