We describe a 74-year-old patient with multicentric Castleman's disease (MCD) whose serum macrophage colony-stimulating factor level was elevated. Serum levels of tumor necrosis factor-alpha and interleukin 6 were also elevated at presentation, and they returned to normal levels after chemotherapy. Although the total serum cholesterol level was below normal on admission, it increased after chemotherapy. These results suggest that the activation of monocytes or macrophages may be involved in certain pathological phenomena in MCD.