One hundred forty-two cases of thyroid carcinomas were studied to assess the frequency with which mucinous deposits occur within the neoplastic cells. In contrast to the literature, which contains only a few scattered reports on this topic, the production of mucin by carcinomas of the thyroid was relatively frequently observed in our material. Almost one half of the cases displayed mucinous substances, which occurred in about 50% of the papillary and the medullary carcinomas, 35% of the follicular types, and about 21% of the anaplastic varieties. The amount of mucinous substances varied considerably: 28 cases were weakly positive, 30 cases were moderately positive, and 6 cases were strongly positive. These findings are discussed with respect to diagnostic and differential diagnostic problems, especially with respect to the interpretation of mucinous deposits that occur in metastatic carcinomata.