Organ dysfunction in AL amyloidosis is related to the production and deposition of amyloidogenic monoclonal light chains. These pathological light chains can now be quantified using the recently developed serum free light chain assay.Methods: We retrospectively reviewed 31 patients with AL amyloidosis to determine the frequency of abnormal free light chain assay results at diagnosis and whether changes in the serum free light chain assay predict outcome after therapy.Results: An abnormal free light chain assay was found in 30 of 31 patients (97%) at the time of diagnosis. In the subset of our patients who received treatment for AL amyloidosis, a >50% reduction of the pathological free light chain following treatment was shown to predict improved overall survival. In our series of analyses, achievement of greater magnitudes of reduction of the free light chain result did not appear to provide additional prognostic information, nor did the baseline free light chain result predict outcome.Conclusion: Our findings support the use of the free light chain assay in the diagnostic work-up of patients with suspected AL amyloidosis, and also as a sensitive biomarker of response to therapy.