Defined by histological criteria, Castleman's disease (CD) is a clinically and histologically heterogeneous syndrome. The functional status of immune cells in affected tissues may vary between the different forms of the disease. To address this question, the expression of cytokine genes in eight CD lymph nodes was analyzed by in situ hybridization. Two lymph nodes were taken from patients with a localized form of the disease associated with systemic manifestations, two from patients with a localized form without systemic symptoms, and four from patients with a multicentric form. Five lymph nodes exhibiting a benign follicular hyperplasia were used as controls. The interleukin-6 (IL-6) gene was expressed at a very high level in two cases: the two localized forms of CD associated with systemic manifestations. IL-6 gene overexpression occurred inside follicles of these lymph nodes. The morphology of follicular cells hybridizing with the IL-6 probe or labeled with an anti-IL-6 monoclonal antibody suggested that follicular dendritic cells expressed the IL-6 gene. In contrast, no IL-6 gene expression was detected inside follicles of the six other CD lymph nodes or of the five control lymph nodes. In interfollicular areas, IL-6 gene-expressing cells were detected in all lymph nodes by both in situ hybridization and immunohistochemistry. In CD lymph nodes, positive cells were located outside sinuses, in close contact with blood vessels and plasma cells. This distribution was clearly different from that observed in control lymph nodes, in which IL-6 gene-expressing cells were present inside sinuses. A similar difference between CD and control lymph nodes was observed for the distribution of IL-1 beta and IL-1 alpha gene-expressing cells in interfollicular areas. The morphology of interfollicular IL-6-producing cells was heterogeneous, consistent with that of macrophages, interdigitating cells, lymphocytes, and endothelial cells, and different from that of plasma cells. Taken together these results show that CD is consistently associated with a particular pattern of IL-6 gene expression in interfollicular areas whereas elevated IL-6 gene expression inside follicles only occurs in the localized form of the disease associated with systemic manifestations. The variable pattern of IL-6 gene expression as well as the clinical and histologic heterogeneity of CD indicate that different immune mechanisms may be involved in the different forms of this disease.