The aim of this study was to clarify the histopathological features of anaplastic thyroid carcinoma in patients who achieved long-term survival. We reviewed 88 anaplastic thyroid carcinoma cases in which the patient survived less than 3 months (short-term survival), and 68 anaplastic thyroid carcinoma cases in which the patient survived more than one year (long-term survival) from the database of the Anaplastic Thyroid Carcinoma Research Consortium of Japan. We examined these cases both histologically and immunohistochemically. Six (6.8%) short-term survival cases and 27 (39.7%) long-term survival cases were considered not to be anaplastic thyroid carcinoma after central review. Of these, 12 were revised to papillary carcinoma with squamous cell carcinoma. In cases without chemotherapy, long-term survival was significantly more common if there was a pre-existing tumor, epithelial growth, or lymphocytic infiltration, and short-term survival was more common if neutrophilic infiltration was present. In cases with chemotherapy, long-term survival was significantly more common if epithelial growth or a squamous cell carcinoma component was present, whereas short-term survival was more common in cases with rhabdoid cells. Immunohistochemical results were not related to survival. Some long-term survival cases showed histological findings other than those typically associated with anaplastic thyroid carcinoma. The presence of a pre-existing tumor, epithelial growth, a squamous cell carcinoma component, no neutrophilic infiltration and lymphocytic infiltration may therefore be favorable prognostic factors in anaplastic thyroid carcinoma.