A patient is presented who had Castleman's disease with constitutional symptoms, a palpable supraclavicular/ axillar mass, and a microcytic anemia, among other laboratory abnormalities, including elevated levels of interleukin-6. Treatment consisted of irradiation of the involved area, with subsequent disappearance of all symptoms and normalization of the laboratory abnormalities. Iron kinetic studies demonstrated a hypoproliferative erythropoiesis, which normalized after radiotherapy. Hypoproliferative erythopoiesis could not be ascribed to serum inhibitors, since normal burst-forming units were observed in the absence or presence of autologous serum. The role of interleukin-6 in relation to Castleman's disease is highlighted.