Castleman's disease (angiofollicular lymphoid hyperplasia) is a heterogeneous group of lymphoproliferative disorders of uncertain cause¹. Two pathologic types, hyaline vascular and plasma-cell disease, have been recognized. The plasma-cell variant of Castleman's disease may be localized or multicentric. Multicentric disease is a systemic lymphoproliferative disorder characterized by lymphadenopathy, hepatosplenomegaly, and constitutional symptoms. Anemia, hypoalbuminemia, and hypergammaglobulinemia are also common. Interleukin-6, a cytokine with pleiotropic effects on the immune system, hematopoiesis, and acute-phase reactions, is a putative growth factor in multiple myeloma and may also be central to the pathophysiology of Castleman's disease²–⁷. Administration of a murine anti-interleukin-6 monoclonal . . .