The relationship between histologic type and survival of 258 thyroid malignancies has been analysed. A new clinicopathologic entity, poorly differentiated carcinoma of the thyroid, is proposed. Papillary and follicular carcinomas of the thyroid showed no significant difference in survival rates. Both tumors were histologically separated into well differentiated and poorly differentiated carcinomas, so that thyroid cancer, deriving from follicular cells, was divided into well differentiated, poorly differentiated, and anaplastic carcinomas. The characteristic histology of poorly differentiated carcinoma was the presence of solid, trabecular and/or scirrhous patterns. Poorly differentiated carcinoma was found in 13.6% of all thyroid malignancies, and its prognosis was worse than that of well differentiated carcinoma. The differences of survival rates among well differentiated, poorly differentiated and anaplastic carcinomas were statistically significant. Prognostic data support the suggestion that the clinicopathologic entity of poorly differentiated carcinoma is of value in determining management and survival of thyroid cancer patients.