Background: Several clinical studies have shown that early treatment of clinically isolated syndrome (CIS), can improve the course and prognosis of the disease. Patients and methods: ATRACT was an observational, non-interventional, prospective, non-comparative, non-randomized, single-arm, open-label, multicentre phase IV study, which primary aim was to investigate a relationship between clinical features of CIS and therapeutic response to 44 pg of subcutaneous interferon (IFN) beta-1a administered three times a week. A total number of 250 subjects aged 18 - 65 years, diagnosed with CB and treated with IFN beta-1a, were enrolled in the study. Patients were followed up for 24 months from baseline and during visits scheduled 6, 12, 18 and 24 months after the baseline visit. Data on the disability level (evaluated by Expanded Disability Status Scale [EDSS]), and number and time of relapses were collected. Results: The proportion of clinically stable subjects was 75.11% within the first year after the treatment initiation, and 59.11% at the end of the follow-up period. Throughout the 2 years, majority of subjects (85.45%) did not experience 3-month confirmed EDSS progression, and the proportion of relapse-free subjects was 62.67%. There was no significant difference in clinical activity between subgroups with mono- and polysymptomatic CB. Conclusion: IFN beta-1a treatment led to a stabilization of clinical activity in most subjects with CIS within 2 years. The clinical stabilization had not been affected by the clinical nature of CIS and the initial MRI finding.