We report a case of anaplastic carcinoma of the thyroid administered peroral fluorinated pyrimidine, providing longterm survival during three years. Three years ago, a 70-year-old woman diagnosed with anaplastic carcinoma of the thyroid was admitted for chemoradiation therapy after tumor resection and tracheostomy. The tumor was confirmed to remain on the margin of resected specimens by postoperative histopathology. Combination chemotherapy, including docetaxel, 5-fluorouracil, cisplatin, and radiotherapy, was performed for four weeks. After completion of radiotherapy, CT scan showed residual tumor and lytic clavicular bone findings. But the patient did not desire the same heavy chemotherapy, wishing rather to undergo chemotherapy as an outpatient, if possible. She received chemotherapy with S-1 which was peroral fluorinated pyrimidine for 9 months. In this period, PET-CT showed that accumulation of FDG and lytic clavicular bone findings were continuously confirmed. She was changed to tegafur-uracil(UFT)from S-1, because of myelosuppression. She is alive at this writing in good physical condition. CT shows no tumor growth and no remote metastasis.