Recently, a highly sensitive assay (FREELITE) capable of measuring serum immunoglobulin-free light chains (FLC) was developed. An abnormal kappa/lambda ratio supports the presence of clonal plasma cell expansion. Using this assay, we measured serum and urine samples of 178 healthy volunteers, 184 patients with polyclonal gamma-globulinemia and 150 patients with monoclonal gamma-globulinemia. The diagnostic sensitivity of the FLC assays for monoclonal gammopathies was 88.0% and the specificity for healthy volunteers and polyclonal gammopathies was 96.1%. The minimal detection sensitivity of this assay for serum FLC was 0.3 mg/l and was greater than 100-fold more sensitive than serum protein electrophoresis (SPE). The combination of FLC with SPE and immunoelectrophoresis identified 99% of patients with monoclonal gammopathies. Effective treatment often leads to a more rapid reduction of the involved FLC level relative to the intact immunoglobulin or total light chain concentration because the half-life of FLC is <6 hours. These observations suggest that FREELITE is useful for diagnosis, disease monitoring and assessment of response to treatment in patients with monoclonal gammopathies such as multiple myeloma and AL amyloidosis.