One hundred twenty-one cases of anaplastic carcinoma of the thyroid treated at M.D. Anderson Cancer Center, Houston, were reviewed. Anaplastic carcinoma is a rapidly growing neoplasm with a dismal prognosis. The mean survival of our patients was 7.2 +/- 10 months. A significant percentage of our patients (35%) had areas of well-differentiated thyroid carcinoma elsewhere, supporting the hypothesis that anaplastic thyroid carcinoma arises from preexisting well-differentiated thyroid carcinoma. Twenty-four of 30 tumors analyzed (84%) stained for keratin, 28 (93.3%) stained for vimentin, and ten (33%) stained for epithelial membrane antigen. Younger patients lived longer than older patients, and patients whose disease was earlier-stage at presentation responded better than patients with metastases at presentation. Radical surgery alone did not significantly increase survival duration over less radical surgery. The role of multimodality therapy needs further evaluation.