We describe a patient with malignant histiocytosis who had a prominent neutrophilia uncommonly observed in this disease. Serum concentration of macrophage colony-stimulating factor (M-CSF) was markedly elevated, and correlated with clinical symptoms and neutrophil counts during clinical course. Moreover, M-CSF was detected in the cytoplasm of malignant histiocytes of the lymph node and bone marrow by immunohistochemical staining. These results indicate that the neutrophilia observed in this patient was caused by M-CSF produced by the malignant histiocytes.