OBJECTIVE Introduction: Systemic lupus erythematosus (SLE) is an autoimmune disease leading to chronic inflammation of numerous tissues and organs. The search for clinically useful markers of its activity is ongoing. At present, it is suggested that serum free light chains (FLC) may be useful in assessing SLE activity. The aim of study: To investigate the relationship between serum levels of free light chains (FLC) and the activity of SLE. METHODS Material and methods: Eighty-four SLE patients (75 female; 9 men) aged 34.9±11.8 years, with the disease duration of 6.2±5.2 years, were included. The disease activity was assessed by: circulating C3 and C4 complement components levels, anti-double-stranded DNA (anti-DNA), SLEDAI-2K scale and levels of FLC. We also assessed the relationship between levels of FLC and clinical manifestations of SLE. RESULTS Results: Median serum levels of FLC κ and FLC λ were 25.9 ± 15.6mg/L and 21.2 ± 9.4 mg/L in SLE pts, respectively. Serum levels of FLC κ were positive in 60 SLE pts (71.4%) and FLC λ in 20 SLE pts (23.8%). The significant correlations were found between levels of FLC κ, FLC λ and of anti-dsDNA (p=0.01; r=0.27); (p=0.001; r=0.35), C3 complement (p<0.02; r= -0.25); (p<0.004; r= -0.31), C4 complement (p<0.04; r= -0.22); (p<0.006; r= -0.3) and SLEDAI -2K (p<0.009; r=0.28); (p<0.001; r=0.35). The SLE pts with arthritis / myositis and hematologic symptoms had significantly higher FLC levels than those without. CONCLUSIONS Conclusion: Measurement of serum levels of FLC can help in the periodical assessment of the disease activity in SLE pts.