Serum free light chain assay is a recently available test for diagnosis and monitoring of patients with plasma cell dyscrasias. In particular, this test is especially useful in patients that were previously difficult to follow with traditional laboratory methods. These patients include non-secretory multiple myeloma, amyloidosis and light chain deposition disease. In addition other uses for the test include monitoring response to treatment and earlier detection of relapse in all patients with plasma cell dyscrasias. Potential uses include assessing progression of patients with monoclonal gammopathy of undetermined significance, smoldering multiple myeloma, solitary bone plasmacytoma and extramedullary plasmacytoma to multiple meyloma. Analytical considerations for the assay are also discussed.