Over the last few years new immunoassays have emerged that allow the measurement of free immunoglobulin light chains (FLCs) in serum to a level of 2-4 mg/l and provide a much greater sensitivity than older methods, such as immunofixation, which is able to detect FLCs at a minimum concentration of 100-150 mg/l. The new FLC assay has enabled the detection of monoclonal protein in some patients with non-secretory myeloma and amyloidosis that were previously undetectable. FLC measurements are quantitative, correlating with disease activity, and are an advance in monitoring light chain only multiple myeloma, AL amyloidosis, non-secretory and oligo-secretory multiple myeloma. Serum FLC concentrations also reflect the disease course in the majority of myeloma patients producing intact monoclonal immunoglobulin proteins and have been incorporated into the new response criteria. The rapid half life of lambda and kappa free light chains means that FLC assays may provide a more rapid indication of the response to treatment but their clinical utility in this setting needs further study. An abnormal FLC ratio has been shown to be a risk factor for progression of monoclonal gammopathy of undetermined significance, smouldering myeloma and solitary plasmacytoma of bone and is prognostic in multiple myeloma.