An 86-year-old male patient was treated by percutaneous ethanol injection therapy (PEIT), following tumorectomy of a papillary thyroid carcinoma and a modified radical neck dissection for its metastasis. After seven treatments with PEIT, the patient was admitted to hospital for a tracheotomy to treat progressive severe dyspnea. Seven days later the patient died from acute renal failure caused by diabetic ketoacidosis. At autopsy, tumor masses were found to comprise mostly scar tissue, and the remaining neoplastic cells often showed anaplastic changes. Fresh coagulation necrosis and hemorrhages were scattered throughout the tumor lesions and their microvessels were occasionally occluded by thrombus. These changes were seen prominently, especially at the central areas of PEIT treatment. Given these findings, we believe that PEIT may be useful in providing local control for the progression of thyroid cancer, especially in cases of unresectable malignant thyroid tumors.