We report herein the case of a 77-year-old woman found to have mixed medullary-papillary carcinoma in the right thyroid with lymph node metastases 30 years after a left thyroidectomy. The preoperative values of serum calcitonin and carcinoembryonic antigen (CEA) were high, and fine-needle aspiration biopsy revealed class V, which led us to suspect papillary carcinoma. A right thyroidectomy with dissection of the right neck lymph nodes was performed. Histopathological examination of the tumor specimens revealed gradual borders between medullary carcinoma and papillary carcinoma with positive immunohistochemical staining to calcitonin, chromogranin A, CEA, and thyroglobulin. The serum levels of calcitonin and CEA decreased to normal after the operation. The point mutation of the RET proto-oncogene was found to be negative by a DNA analysis of the peripheral leukocytes. This cancer seemed not to be associated with multiple endocrine neoplasia type 2 syndrome. The presence of both medullary and papillary components in the thyroid with lymph node metastases is rare and may suggest that the tumor had arisen from a common stem cell.