We report a resected case of ascending colon cancer with left supraclavicular and paraaortic lymph nodes and liver metastases which completely responded in terms of metastases but not the primary tumor to FOLFOX4 therapy. A 62-year-old woman with epigastric discomfort was initially diagnosed as malignant lymphoma by FDG-PET with abnormal accumulation at left supraclavicular and paraaortic lesions. Pathological examination of the supraclavicular lymph nodes showed undifferentiated adenocarcinoma, and ascending colon cancer was detected by colonoscopy which was a mixture of various types of differentiation. FOLFOX4 therapy was effective for metastatic lesions but colon tumor did not regress and was accompanied by abdominal pain. Macroscopically, a curative right hemicolectomy was performed, and microscopic examination revealed that the tumor had become a mass of undifferentiated cancer cells. Thus, the present case demonstrates the dedifferentiation of colon cancer during chemotherapy.