This paper describes an extremely rare case of the simultaneous occurrence of medullary and papillary thyroid carcinoma. A 31-year-old woman had two different coexistent tumors, i.e., medullary carcinoma (1.2 X 1.5 cm) adjacent to papillary carcinoma (2 X 1.8 cm) in the right lobe. The two different tumor cells revealed the characteristic features of each on the microscopic, histochemical and ultrastructural findings. Therefore, they must be derived from different origins. One lymph node contains metastases of mixed medullary and papillary carcinoma. Such an unusual case of concurrent medullary and papillary carcinoma seems to be interesting as to the histogenesis.