Most cases of anaplastic thyroid carcinoma can be pathologically and often historically associated with the presence of low-grade (differentiated) cancer in the thyroid. That radiation therapy to the differentiated tumor plays an etiologic role in the transformation of a differentiated to an undifferentiated tumor has been suggested. If such therapy can be implicated, is there a difference in risk between external radiotherapy or radioactive iodine? Review of the literature discloses that more anaplastic carcinoma of the thyroid develop in patients without a history of prior radiation than in individuals who have received radiation. We report our recent experience with two patients who demonstrated the sequence of well-differentiated followed by anaplastic thyroid cancer subsequent to radiation and review the question.