BACKGROUND: The Freelite system for nephelometric or turbidimetric measurement of serum free light chains (FLCs) has been available since 2001. It has been valuable for the management of patients with oligosecretory myeloma, light chain myeloma and AL amyloidosis. However, there are several limitations of the method. The goal of this study was to evaluate the analytical performance of the FLC assay. METHODS: Titrated controls and clinical serum specimens were used to determine precision and post-dilution recovery. RESULTS: As reported elsewhere, we found that the assay had several limitations, including poor post-dilution linearity and overestimation by nephelometry. CONCLUSIONS: These data demonstrate that the results of the FLC assay must be interpreted jointly by the clinician and the biologist, taking into account the individual patient's clinical and biological characteristics.