Prognosis of well-differentiated carcinoma of the thyroid gland is generally favorable, while that of anaplastic carcinoma, extremely poor. Well-differentiated carcinoma may sometimes be fatal; the most common underlying cause is considered to be due to anaplastic transformation of the original well-differentiated carcinoma to a less differentiated form. We studied 27 consecutive autopsy cases of fatal thyroid cancer treated at the Ito Hospital, Tokyo, during a five-year period, 1969-1973. We found uniform histological features of anaplastic carcinoma in 10 cases and of well differentiated carcinoma in four cases. In addition, co-existence of well-differentiated and anaplastic carcinomas was observed in nine cases and well differentiated and squamous cell carcinomas in four. Circumstantial evidence strongly suggests that malignant transformation is a part of the natural history of thyroid carcinoma, from well-differentiated carcinoma to less differentiated forms, either squamous cell or anaplastic carcinoma.