Identification of patients at risk of early disease progression is the mainstay of tailored management in chronic lymphocytic leukemia (CLL). Although application of established biomarkers is limited by intrinsic detection/readout complexities, abnormality of κ and λ serum-free light chain ratio [sFLC (κ/λ)] was proposed as a straightforward prognosticator in CLL. By analyzing 449 therapy-naive patients, we show that an abnormal sFLC(κ/λ), along with CD38, ZAP-70, IGHV mutations, cytogenetics and stage, independently predicts treatment-free survival (TFS) but becomes prognostically irrelevant if the cumulative amount of clonal and nonclonal FLCs [sFLC(κ + λ)], a variable associated with cytogenetic risk, exceeds the threshold of 60.6 mg/mL. Patients with sFLC(κ + λ) above cut-off displayed a poorer TFS outcome, irrespective of sFLC(κ/λ). Only ZAP-70, cytogenetics, stage, and TFS remained associated with sFLC(κ + λ) in a multivariate model. By assigning 1 point each for these variables, the 3-year probability of TFS was 94.8%, 84.5%, 61.6%, and 21.1% for patients scoring 0, 1, 2, and 3 + 4, respectively (P < .0001). These data, and the demonstration that monoclonal and polyclonal B cells concur to FLC synthesis in tumor tissues, suggest that sFLC(κ/λ) and sFLC(κ + λ) mirror distinct biologic processes in CLL. sFLC(κ + λ) assessment represents a sensitive and cost-effective tool for identifying CLL patients requiring early treatment.