We analyzed the clinicopathologic and therapeutic factors associated with long-term survival in 449 patients with anaplastic carcinoma registered with the Anaplastic Thyroid Carcinoma Consortium of Japan. Univariate analysis showed a significant relationship between long-term survival of more than 1 year and the following factors: the appearance of acute symptom; WBC < 10,000/mm3, tumor diameter < 5.0 cm; tumor remnant in the thyroid gland after surgery; no distant metastasis at initial diagnosi; complete resection of gross tumor; and administration of external irradiation (40 Gy) and chemotherapy. In multivariate analysis, only tumor diameter (< 5.0cm), complete resection of gross tumor, and external irradiation (40 Gy) showed a significant correlation with long-term survival. Only 31 of the 449 patients (7%) had these 3 factors, and 20 (65%) of them survived for more than 1 year. Among patients in any stage or with any prognostic index score who received little treatment, very few survived for more than 1 year. In contrast, many patients who received intensive combination therapy survived for more than 1 year. The results of this study suggest that sufficient intensive combination therapy has practical significance for patients with anaplastic carcinoma, except for those with advanced-stage disease.