Reported is the case of a 70-year-old woman who, on diagnosis, was found to have a papillary carcinoma of the thyroid gland that showed an anaplastic transformation of a metastatic lesion of the right cervical lymph node. The primary lesion, however, contained no anaplastic areas. Simultaneously, she also was found to have a primary carcinoma of the left breast, an invasive ductal carcinoma. She thus received a total thyroidectomy with a right cervical node dissection, a modified radical mastectomy, and irradiation of the right cervical area, but no intensive chemotherapy was performed. Six months after the thyroidectomy, the anaplastic carcinoma caused her death. On autopsy, it was found to have involved the right cervical area, the lungs, the mediastinum, the right axilla, and the right kidney.