<b>Introduction:</b> There are two commercially available tests for measurement of serum free light chains (sFLC) in multiple myeloma (MM) patients – Freelite and N Latex FLC. The aim of this study was to perform an assessment and direct comparison of the usefulness of the methods in routine clinical practice.<br/><b>Methods:</b> 40 refractory/relapsed MM patients underwent routine disease activity assessment studies, along with sFLC analysis using both assays. Correlation and concordance between the tests and sensitivity of studied methods of sFLC assessment were established. Special attention was focused on sFLC results in patients finally evaluated after completing the treatment. <br/><b>Results:</b> A weak correlation for the measurement of both κ [Passing–Bablok slope (PB) = 0.7681] and λ chains [(PB) = 1.542] was found. Using Bland–Altman plots, a bias of 0.0467 (κ) and -0.2133 (λ) between the measurements was documented. The concordance coefficient equaled 0.87 for κ, 0.62 for λ and 0.52 for κ/λ ratio. Ten patients had an abnormal Freelite assay κ/λ ratio and normal N Latex FLC κ/λ ratio. Three of these patients had negative serum protein electrophoresis results and fulfilled diagnostic criteria of stringent complete remission (sCR) according to N Latex FLC (but not according to Freelite). When the κ/λ ratio obtained by both methods was compared to patients’ serum/urine protein electrophoresis and immunofixation results, sensitivity of Freelite and N Latex FLC was established to be 62.5% and 41%, respectively. <br/><b>Conclusions:</b> There was no strong correlation or concordance between the two assays, and the sensitivity in terms of sFLC detection was different. This may cause problems when diagnosis of sCR is considered.