We report a rare case of granulocyte-colony stimulating factor (G-CSF)-producing undifferentiated carcinoma of the ascending colon. A 52-year-old Japanese man presented with a rapidly growing, aggressive abdominal tumor, and severe leukocytosis (63 000/mm(3)). The serum level of G-CSF was remarkably elevated to 640 pg/ml (normal, <18.1 pg/ml). The patient underwent palliative cytoreductive surgery for ascending colon carcinoma with lymph node and liver metastases. Histological examination revealed an undifferentiated carcinoma of the ascending colon. The tumor cells were positive for G-CSF on immunohistochemical staining. The leukocyte counts and G-CSF level decreased after surgery. Thus, we diagnosed G-CSF-producing colon carcinoma. His general condition deteriorated rapidly and he died of residual tumor growth on postoperative day 24.