Monoclonal free light chains are found in the serum and urine of patients with B-cell proliferative disorders, including multiple myeloma. Measuring free light chains in serum is useful for the diagnosis and monitoring of free light chains diseases. Moreover it could be interesting in the monitoring of treated multiple myeloma with complete immunoglobulin and of monoclonal gammapathy of undetermined significance (MGUS). The goal of our work was to analyze a large cohort of patients with multiple myeloma or MGUS from January 2003 to August 2006 in order to better understand the interest of free light chains.