The detection of IgG oligoclonal bands (OCBs) in cerebrospinal fluid (CSF) is, as yet, the recommended biochemical marker for the diagnosis of multiple sclerosis (MS). Aim of this study was to investigate the behaviour of free light chains (FLC) in OCBs negative (OCBs-) MS patients compared with that in OCBs positive (OCBs+) MS patients and in a control group (CG) of subjects without cerebrospinal inflammatory disease. At multiple comparisons between the three groups, statistically significant differences (p < .001 for all) were found for κFLC. Conversely, λFLC values evidenced a greater overlapping in the three groups. Receiver operating characteristics (ROC) curves made with κFLC values, evidenced the greater differences of areas under curves (AUCs) between OCBs- and OCBs+ (AUCs: κFLC 0.98, QκFLC 0.98, κFLC index 0.96) with respect to the differences between OCBs- and CG (AUCs: κFLC 0.77, QκFLC 0.86, κFLC index 0.77): indeed >50% of MS OCBs- subjects studied evidenced the same values of κFLC, QκFLC and κFLC index found in CG. Conversely, if the aim is to select MS subjects while avoiding undertaking the more complex isoelectrofocusing test, values with absolute specificity for MS (QκFLC = 15, sensitivity = 0.76 and κFLCindex = 3.09, sensitivity = 0.72) could be used. The values found in this study call for confirmation with data from more subjects, including those with other CSF inflammatory diseases. Anyway, the most important finding was that, for some OCBs- subjects, κFLC are more effective than OCBs in diagnosing MS.