An unusual case of poorly differentiated carcinoma of the thyroid is reported. The tumor occurred in a 60-year-old man, who died with widespread metastases 5.5 years after primary treatment. The unencapsulated tumor measured 8 X 5 X 9.5 cm and was composed of columnar cells. Two to three mitotic figures per 10 high-power fields were present. The cells were thyroglobulin positive and not stained for calcitonin and carcinoembryonic antigen (CEA). The light microscopic appearance was similar to that of a metastasis from a carcinoma of the bowel. The authors conclude that columnar cell carcinoma is a separate type of thyroid carcinoma that shares the clinical properties of both follicular and papillary carcinoma and carries a poor prognosis. However, for practical purposes, the authors think that the tumor should be included in the category of poorly differentiated thyroid carcinomas together with the so-called insular carcinomas, mucoepidermoid carcinomas, and mucinous carcinomas.