The condition of making the diagnosis of tick-borne encephalitis is the performance of serological investigation, quite apart from careful evaluation of medical history and clinical status of patient. Contemporary diagnostics of tick-borne encephalitis uses immuno-enzymatic test (ELISA), which enables detection of specific IgM antibodies (in initial phase of disease) or IgM and IgG antibodies (in the phase of the disease with neurological symptoms). The highest diagnostic value in the infection phase of disease has the demonstration of at least fourfold increase of viral antibodies level, measured in acute phase and 2 to 6 weeks after that. In case of multiple sclerosis, a laboratory test, specific for the illness, has not been established as yet. The basis for establishing the diagnosis is the clinical picture. Results of auxiliary examinations are used for the completion of clinical evaluation and for differential diagnosis. Seroepidemiological investigations were carried out in multiple sclerosis patients to estimate the level of antibodies against tick-borne encephalitis. The investigations indicated low level of IgG antibodies in patients' serum.