Background: fingolimod is a drug administered orally to adult patients treated for relapsing remitting course of multiple sclerosis (Ms). Mode of action of fingolimod is based on intense s1P1 receptor stimulation and “arresting” lymphocytes in lymphatic organs. objective of the research was to assess changes in the frequencies of basic lymphocyte subsets in patients treated for multiple sclerosis with the use of fingolimod. Material and methods: study group comprised of 25 previously untreated adult patients with M s. Venous blood samples were collected from each patient before and one month, three months and six months after treatment initiation. Peripheral blood lymphocyte immunophenotype was assessed with a set of monoclonal antibodies bounded to appropriate fluorochromes and flow cytometer facscalibur. statistical analysis of the results was conducted using statistica 9.0 software. Results: before fingolimod administration median of lymphocyte subsets percentage in each patient was in reference range. after 1 month of treatment we noticed significant changes in frequencies of following lymphocyte subsets: nk cells – 51.22% (p = 0.016), t cD4+ cells – 11.58% (p = 0.01), t cD4+:t cD8+ cells ratio – 0.61 (p = 0.005). after 3 and 6 months of treatment there was further increase of deviation from normal state. Conclusions: the use of fingolimod is associated with profound changes in lymphocyte subsets distribution, which might bear a risk of the development of cellular immune deficiency symptoms.