Serum free light chain (sFLC) assays were shown to improve detection, management, and prognostication in plasma cell disorders. Recently, sFLC assays improved detection of M proteins when combined with standard methods of protein electrophoresis/immunofixation in patients with non-Hodgkin lymphoma/chronic lymphocytic leukemia (NHL/CLL). Incidence of abnormal sFLC ratio (sFLCr) varied from 0% to 36% and 29.7% to 59% in NHL and CLL, respectively. Increased sFLC levels or abnormal sFLCr predict shorter overall survival in early-stage CLL. Furthermore, abnormal sFLCr correlated with advanced disease stage and poorer outcome. In diffuse large B-cell lymphomas, increased sFLC was demonstrated as an independent, adverse prognostic factor for overall/event-free survival. Moreover, abnormal sFLCr can be a diagnostic tool in central nervous system lymphomas. Finally, the quantitative FLC assay has the potential to become a new, easily measured biomarker for predicting prognosis and enhanced detection in NHL/CLL. It may be used serially at follow-up evaluations to provide clues to relapse.