Anaplastic carcinoma of the thyroid is generally considered a primary condition. Our study, based on ten observations, shows that anaplastic carcinoma of the thyroid with a rapidly fatal course may arise in long-standing goiters. Borderline histologic changes may be encountered; in some differentiated nodules, undifferentiated areas were found. Factors inducing transformation is equally obscure. Anaplastic changes may result from the growth of previously quiescent clones. The time-interval between benignant and malignant lesions varies widely and is difficult to establish unequivocally. In some instances, anaplastic carcinoma occurs after surgical removal of differentiated carcinoma.