Objective. To evaluate safety and efficacy of natalizumab (tisabri) in the RUSTYS study of patients with relapsing-remitting multiple sclerosis (RRMS) during one year. Material and methods. This prospective, open, non-randomized clinical study included 100 RRMS patients treated with natalizumab in dose 300 mg intravenously every 4 weeks during 48 weeks. The safety was assessed by the percentage of patients with infusion reactions as well as by the distribution of undesirable effects (UE). Before treatment and after 48 weeks, antibodies to JC virus (JCV) were determined. The efficacy was evaluated by the frequency of relapses, progression of disability, MRI results and quality of life indices. Results. After treatment, 87% of the patients had no relapses, disease progression assessed by EDSS was not observed in 96% . To 48th week, 71% of patients had no MRI changes in T2 lesions and 79.6% — in T1 hypointense lesions. Quality of life has been improved. At least one UE, including the development of tuberculosis in one patient, was noted. Infections (upper respiratory tract infection, nasopharyngitis, flu) were recorded in 25% of the patients. Three out of 28 (10.7%) of the patients with negative results of JCV analysis at baseline had the seroconversion with the change of the negative status to the 48th week. No cases of progressive multifocal leukoencephalopathy related to natalizumab were found. Conclusion. The results are in line with those of earlier studies in patients with RRMS with high disease activity.