Plasma cell dyscrasias can cause renal disease. Sensitive methods have recently been introduced to quantify serum free light chains (sFLCs). Renal function may influence the variability of these methods, as shown in chronic kidney disease (CKD) patients, but this problem has not been widely addressed in renal transplant patients. Herein, we examined the association between polyclonal sFLC concentrations and renal function among a population of renal transplant patients. We studied 102 kidney allograft recipients and 53 CKD patients classified according to KDOQI (Kidney Disease Outcomes Quality Initiative) stages. None of them had been diagnosed with monoclonal gammopathy. sFLCs were quantified by nephelometry. Both serum κ and λ free light chain concentrations rose progressively through each stage of KDOQI among both transplant and nontransplant patients (P<.0001). In the former setting, sFLC concentrations significantly correlated, using a Spearman coefficient, with serum creatinine, and serum cystatin concentrations as well as estimated glomerular filtration rate: namely, 0.723, 0.797, and -0.711 for sκFLC and 0.705, 0.759, and -0.694 for sλFLC, respectively (P<.0001 in all cases). Spearman correlation coefficients in nontransplant patients were: 0.559, 0.848, and -0.766 for sκFLC and 0.702, 0.875, and -0.855 for sλFLC, respectively (P<.0001 in all cases). In conclusion, sFLCs must be interpreted cautiously due to their clear association with renal function. Therefore, renal transplantation did not produce changes that were different from those dependent on renal function.