This report presents a rare histological variation of medullary (C-cell) carcinoma of the thyroid, referred to as giant cell type, which is similar to that found in anaplastic carcinoma of the thyroid, or choriocarcinoma. The giant cells in this case possessed specific immunofluorescence for calcitonin and secretory granules in the cytoplasm. The giant cells were tumor cells of medullary carcinoma and could be distinguished from anaplastic carcinoma of the thyroid by various histological characteristics, such as nuclear invagination, an intermingled pattern of giant cells with typical small solid cells, infrequency of mitosis, and the existence of amyloid stroma.