Background: Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) with the first relapse recorded as clinically isolated syndrome (CIS). The diagnostic primarily includes MRI findings and demonstration of intrathecal synthesis of oligoclonal IgG bands with a qualitative method of isoelectric focusing (IEF) with immunofixation. Recent studies have shown high concentrations of free light chains in CSF in these patients, especially kappa type (KFLC). The objective of the study was to evaluate diagnostic and predictive value of determination of KFLC in CSF compared to the qualitative procedure of determination of oligoclonal bands (OB) in patients with CIS who have converted to MS during a two-year period. Methods and patients: For 151 patients with suspected CIS, who were admitted to Clinic of Neurology, concentrations of KFLC, total IgG and albumin in CSF and serum were measured by an immunonephelometric method. CSF/serum quotients (QKFLC, QIgG and QAlb) and indices KFLCindex (QKFLC/QAlb) and IgGindex(QIgG/QAlb) were also calculated with regards to albumin, and the presence of OB was assessed with isoelectric focusing. Based on the clinical picture, OB and MRI findings, 50 patients were classified as „other neurological diseases (OND)“ and 101 patients were classified as CIS, 50 of which converted to MS (D-MS) in a two-year period. Results: An optimal cut-off was determined with analysis and comparison of ROC curve, and with median comparison of KFLC parameters, with regards to CIS diagnosis and conversion to MS. ROC analysis for accuracy of intrathecal synthesis showed the best results for QKFLC with AUC of 0.891 and cut-off of 0.027 with the diagnostic sensitivity of 73.2 % and specificity of 96.0 %. The diagnostic accuracy of KFLCindex for conversion of CIS to MS showed AUC of 0,840 and cut-off of 9.092. QKFLC values below 0.0148 and above 0.0270 indicate needlessness for a qualitative, technically demanding method of IEF with immunofixation for demonstration of OB. KFLCindex value below 4.44 in patients with CIS does not indicate conversion to MS inside a two-year period while value of KFLCindex above 104.54 is very indicative of this diagnosis. Conclusion: The application of KFLC parameters could serve as a screening method, and OB could be used in undefined clinical cases only. KFLC parameters are significantly higher in patients who converted to MS, and can contribute to an early diagnosis and prompt therapy with their predictive role.