We examined the case of a 52-year-old man presenting with a sarcoma accompanied by severe leukocytosis, which developed many years after a femoral fracture. Histologic, histochemical, immunohistochemical, and ultrastructural analysis of the tumor revealed that the sarcoma could not be classified by any of the bone or soft tissue tumor classifications currently in use. The tumor cells were isolated from surgical specimens and subcultured in vitro. The concentration of granulocyte colony-stimulating factor in the culture medium was constantly elevated to considerably high levels during 20 cell passages. A genomic study using reverse transcription-polymerase chain reaction showed that the cells retained messenger RNA expression of granulocyte colony-stimulating factor. The aberrant overexpression of granulocyte colony-stimulating factor clearly represented a paraneoplastic phenomenon of the neoplastic cells.