['We hypothesized that surrogate markers of host immune response may predict survival in myeloid malignancies. Because of immediate practical applicability, we chose plasma immunoglobulin free light chain (FLC) concentration as the biomarker of interest.', 'Two independent cohorts of patients with primary myelofibrosis (PMF) or myelodysplastic syndromes (MDS) were studied. Kappa (κ) and lambda (λ) FLCs were measured by a quantitative nephelometric assay. Patients with monoclonal FLC were excluded.', 'Values that were above the upper limit of normal for κ or λ FLC were documented in 33% of 240 patients with PMF and 46% of 74 patients with MDS. Increased FLC was significantly associated with increased creatinine, and advanced age in PMF (P < .001) and hemoglobin less than 10 g/dL in MDS (P = .005). In multivariable analysis, increased FLC predicted shortened survival in both PMF and MDS, independent of age, creatinine, and other conventional risk factors. Cutoff levels based on receiver operating characteristic analysis for κ plus λ total FLCs delineated risk groups with highly significant differences in overall survival; International Prognostic Scoring System-adjusted hazard ratio in PMF was 1.9 (95% CI, 1.3 to 2.7), and was 6.3 (95% CI, 2.7 to 16.6) in MDS. No correlations were seen with leukemia-free survival, karyotype, or JAK2, MPL, or IDH mutations. In patients with PMF who were studied by cytokine profiling, the prognostic value of an increased FLC level was independent of that from circulating interleukin-2 receptor (IL-2R) or IL-8 levels.'] Do polyclonal immunoglobulin free light chain levels predict survival in myeloid neoplasms?