POEMS syndrome is a multisystemic disorder characterized by the association of polyneuropathy, organomegaly, endocrinopathy, M protein, skin changes and various other systemic clinical signs. The pathophysiology of this syndrome remains largely unknown. In order to gain insight into its pathophysiology, we studied the clinical characteristics and performed serum analysis (auto-antibodies, cytokine levels) and phenotypic and cytogenetic studies of bone marrow plasma cells (BMPC) in six patients with unequivocal POEMS syndrome. Two unusual clinical signs were present in these patients: pulmonary hypertension (two patients) and diffuse cutaneous necrosis (one patient). No auto-antibodies against peripheral nerve (PN) antigens (SGPG and SGLPG glycolipids, GM1, GD1a, GD1b and GT1b gangliosides) were found. Sequential evaluations of serum cytokines (IL-1-beta, IL-6 and TNF-alpha) showed a moderate to marked elevations of IL-6 and TNF-alpha in all patients (up to six-fold for TNF-alpha and 16-fold for IL-6). Using in situ hybridization of these cytokines mRNAs on lymph node specimens of two patients who had an angiofollicular lymph node hyperplasia, a strong positivity was found with the IL-1-beta antisense probe in lymph node macrophages. On skin biopsy a high number of cells expressing TNF-alpha mRNA was observed in the dermis. The biological features of BMPC: phenotype (expression of CD19 and CD56 antigens), kinetics (Ki-67 index), karyotype, DNA content and chromosomal in situ hybridization remained those of BMPC found in monoclonal gammopathy of undetermined significance. We conclude that POEMS syndrome is a hypercytokinemic syndrome in which BMPC are not of malignant type. Macrophages are involved in this syndrome and their role has to be further investigated as well as treatments which act through an anti-cytokine mechanism.