POEMS (polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy, and skin changes) syndrome is a rare plasma cell dyscrasia. Nearly all patients present with a λ-restricted monoclonal gammopathy. Most patients with POEMS syndrome have been reported to have a normal serum free light chain ratio (sFLC-R), but the underlying mechanism is still unclear. We assessed the serum free light chains in 83 patients with newly diagnosed POEMS syndrome. The clinical and laboratory data associated with this disorder were collected to identify factors affecting sFLC-R. Fifty-six patients (67 %) showed elevated serum free λ light chains, but only 11 patients (13 %) had an abnormal sFLC-R. A comparison of patients with and without abnormal sFLC-Rs indicated that the latter group had more common splenomegaly and worse renal function. However, the introduction of an extended renal range for sFLC-R did not dramatically improve the diagnostic value of sFLC-R in these patients. Further analyses identified a correlation between the serum free κ light chain and the uninvolved immunoglobulin in patients with an IgAλ clone, implying that the activation of polyclonal immunoglobulin production could mask the presumed skewing of the sFLC-R induced by the underlying monoclonal gammopathy. Therefore, a serum heavy/light chain (sHLC) assay was performed in a subset of patients with stored serum samples available, and the prevalence of abnormal sHLC ratios was high in these patients. In summary, the overproduction of polyclonal immunoglobulin accounts for the high frequency of normal sFLC-R in patients with POEMS syndrome. The sHLC assay may provide unique information about this disorder.