Castleman's disease is a syndrome consisting of giant lymph node hyperplasia with plasma cell infiltration, fever, anemia, hypergammaglobulinemia, and an increase in the plasma level of acute phase proteins. It has been reported that clinical abnormalities disappear after the resection of the affected lymph nodes, suggesting that products of lymph nodes may cause such clinical abnormalities. Interleukin-6 (IL-6) is a cytokine inducing B-cell differentiation to immunoglobulin-producing cells and regulating biosynthesis of acute phase proteins. This report demonstrates that the germinal centers of hyperplastic lymph nodes of patients with Castleman's disease produce large quantities of IL-6 without any significant production of other cytokines. In a patient with a solitary hyperplastic lymph node, clinical improvement and decrease in serum IL-6 were observed following surgical removal of the involved lymph node. There was a correlation between serum IL-6 level, lymph node hyperplasia, hypergammaglobulinemia, increased level of acute phase proteins, and clinical abnormalities. The findings in this report indicate that the generation of IL-6 by B cells in germinal centers of hyperplastic lymph nodes of Castleman's disease may be the key element responsible for the variety of clinical symptoms in this disease.