We recently encountered an unusual carcinoma of the thyroid gland with a predominantly medullary pattern admixed with areas of follicular differentiation. Both patterns prevailed at the primary site and in bilateral cervical lymph node metastases. The tumor cells were stained for calcitonin by indirect immunofluorescence technique and were found to contain dense-core granules by electron microscopy. Calcitonin was demonstrated in tumor homogenates by radioimmunoassay and was elevated in the serum. Immunofluorescence staining also revealed thyroglobulin in the neoplastic cells. Moreover, following total thyroidectomy, the cervical node metastases concentrated radioactive iodine (131I), and serum thyroglobulin was increased at one stage of the disease when measured by radioimmunoassay. These findings are discussed in the light of the dual embryonic derivation of the thyroid gland.