OBJECTIVE To assess an impact of immunoglobulin free light chains (FLC) on short-term and long-term prognosis of clinical and radiological activity and progression of disability in multiple sclerosis (MS). METHODS A sample of 381 patients with definite MS was divided into 2 groups. In group 1, lumbar puncture was performed at the time of clinically isolated syndrome, and patients were prospectively followed up to 2 years (short-term prognosis group, n=97). In group 2, MS was diagnosed immediately after lumbar puncture, and retrospective analysis of the disease course with the duration not less than 5 years was performed (long-term prognosis group, n=284). The Expanded Disability Status Scale (EDSS) and the Multiple Sclerosis Severity Score (MSSS) were used to assess patient's status. Concentrations of kappa and lambda FLC in the CSF (K-FLCCSF, L-FLCCSF) and serum (K-FLCSERUM, L-FLCSERUM) as well as quotients of concentrations (Q-K and Q-L) were determined. Patients were stratified into subgroups with high and low concentrations of K-FLC and L-FLC using cut-offs from our previous studies: K-FLCCSF=0.595 mcg/l and L-FLCCSF=0.127 mcg/l. RESULTS In group 1, significant correlations were found only between EDSS score and concentrations of K-FLCCSF (r=0.377, p=0.00019) and Q-K (r=0.366, p=0.0012). FLC concentrations did not correlate with the number of relapses and new T2 lesions. The age and EDSS score at the disease onset didn't differ between patients with high and low K-FLC and L-FLC (K-FLCCSF: р=0.2658; L-FLCCSF: р=0.5502). A significant decrease of EDSS score after the disease onset was observed in all groups except for patients with high concentrations of K-FLCCSF (p=0.1844), so the EDSS score after 2 years was significantly higher in this subgroup of patients (p=0.0006). In group 2, significant correlations of K-FLC with EDSS score (r=0.181, p=0.002) and MSSS score (r=0.121, р=0.044) for long-term prognosis (median (IQR) = 8 (6-13) years) were found. No correlations of FLC concentrations with the number of relapses during the first 5 years were found. Survival analysis showed that high concentrations of K-FLCCSF were associated with the high risk of progression to EDSS 6 (HR=2.055, p=0.026) but not with EDSS 4 (HR=2.388, p=0.08). CONCLUSIONS Concentrations of kappa FLC can help to define the prognosis of MS early at the disease course. Although low concentrations of FLC do not exclude a severe disease phenotype, patients with high K-FLCCSF concentrations are at greater risk for faster MS progression, probably, due to impaired reparation of neural tissue. Measurement of FLC concentrations can be used to determine a therapeutic tactics in patients with MS.