Fourteen cases of anaplastic carcinoma of the thyroid (ACT) were studied with silver staining and immunohistochemical technics for signs of medullary differentiation. For comparison a series of 11 cases of medullary carcinoma of the thyroid (MCT) was studied. In nine out of 14 cases of ACT both argyrophilia and immunoreactive calcitonin could be detected as evidence of an apparent C-cel origin of these tumors. Most of the patients with these anaplastic medullary carcinomas died within 6 months after the histological diagnosis was made. It is concluded that the histological diagnosis of anaplastic variants of MCT can only be made on the basis of argyrophilic cytoplasmic granules and calcitonin immunoreactivity. In contrast to well differentiated MCT the anaplastic medullary carcinoma is characterized by a poor prognosis.