A quantitative nephelometric assay for free light chains (FLCs)  has recently been introduced as a commercial test. The assay measures [kappa] and [lambda] light chains that circulate as light chain monomers or dimers and are not bound to immunoglobulin heavy chain. Quantification of the is and FLCs and calculation of the FLC [kappa]/[lambda] ratio have been reported to be sensitive and specific for detection of excess monoclonal FLCs. We have recommended a diagnostic range for the FLC [kappa]/[lambda] ratio that included 100% of a 282-sample reference population to maximize the diagnostic specificity and minimize false-positive results (1). Retrospective studies using stored serum from populations of patients with nonsecretory multiple myeloma (NSMM) (2), primary systemic amyloidosis (AL) (3, 4), light chain deposition disease (LCDD) (1), and light chain multiple myeloma (LCMM) (5) have documented the sensitivity of these assays and established their use as a complement to immunofixation electrophoresis (IFE). In addition to its diagnostic use in the FLC diseases, the assay is used for monitoring disease course in AL, LCDD, NSMM, and LCMM, in which there may be a band detected on IFE that cannot be quantified by protein electrophoresis. Our clinical laboratory implemented FLC testing in late 2002. In 2003, we performed FLC assays on 1020 samples from Mayo Clinic patients. These patients were seen predominantly by clinicians in the Division of Hematology. To assess the performance of the FLC assay in our routine clinical laboratory practice, we reviewed the diagnoses and FLC results for these 1020 patients.