To evaluate a possible implication of cytokines in the pathogenesis of polyneuropathy, organomegaly, endocrinopathy, M protein, and skin changes (POEMS) syndrome, we studied five consecutive patients with this condition, of which four had sclerotic bone lesions and four had multicentric Castleman's disease. Interleukin-1 beta (IL-1 beta) and IL-6 serum levels were determined in these patients (13 serum samples) and in patients with multiple myeloma (5) and Waldenström's macroglobulinemia (5). In situ hybridization of the relevant mRNAs was performed on lymph node specimens of two patients with POEMS syndrome who had Castleman's disease. Elevated serum levels of IL-1 beta (13/13 samples), and IL-6 (7/13 samples) were found in patients with POEMS syndrome. In the other patients, serum IL-1 beta was undetectable or slightly increased and IL-6 was elevated in a single patient with Waldenström's macroglobulinemia. Abundant IL-1 beta mRNA-producing cells were present in interfollicular spaces in the two tested patients, while IL-6 mRNA-producing cells were rare. We conclude that IL-1 beta and IL-6 serum levels may be chronically elevated in patients with POEMS syndrome, and that lymph node may be one site of IL-1 beta overproduction. These results are in keeping with the hypothesis that cytokines mediate systemic manifestations of POEMS syndrome.