Systemic lupus erythematosus (SLE) is a systemic autoimmune disease. Lupus nephritis (LN) affects 30% to 60% of adult Mexican patients. This study aimed to determine whether immunoglobulin free light chains (FLCs) constitute a biological marker for LN. FLCs have been previously studied as disease activity biomarkers, but patients with kidney damage are frequently excluded. A total of 75 consecutive SLE patients were recruited from a third-level hospital in Mexico City. Thirty-eight patients were diagnosed with LN, and the remaining 37 patients without kidney failure were included as a control group. FLCs concentration was measured by nephelometry. The results showed that SLE patients with high FLCs concentration had a higher rate of kidney failure and that the probability of renal failure increased by 2.4% for each mg/dL of κ FLCs, and 1.7% for each mg/dL of λ FLCs. This study suggests that high FLCs concentration is associated with LN, and therefore may be considered as a possible biomarker for SLE activity.