The authors report a case of Castleman's disease in which the major clinical problems have been caused by severe thrombocytopenia and neutropenia. The cytopenias were caused by autoimmune destruction of mature cells as demonstrated by bone marrow findings and cell-specific antibody assays. There also was evidence of red cell autoantibody production but no overt hemolysis. The thrombocytopenia currently is controlled by intermittent vincristine, having proved refractory to steroids, splenectomy, and danazol and only partially responsive to high dose intravenous immunoglobulin. The neutropenia showed temporary improvement with immunoglobulin but has been resistant to all other therapy. This case provides further evidence for disordered immune regulation in Castleman's disease.