PURPOSE: Anaplastic thyroid carcinoma is characterized with rapid growing mass of the neck and early infiltration into the surrounding tissue. Because of its advanced presentation in elderly patients with poor general condition, difficulty in diagnosis and lack of effective treatment, it is one of the most lethal cancers in human. The aim of this study was to investigate the clinicopathologic character of anaplastic thyroid carcinoma and analyze the prognostic factors affecting survivals. METHODS: Twenty patients diagnosed anaplastic thyroid carcinoma in Seoul National University Hospital between 1985 and 1999 were reviewed retrospectively. RESULTS: The most common symptom was a rapidly enlarging neck mass. Ten (55.5%) of 18 patients had a concomitant well differenciated thyroid carcinomas and 5 (27.8%) patients had benign thyroid disease on pathologic features. The median survival was 5.5 months and the 2-year survival rate was 27.3%. Among several factors analyzed, tumor size smaller than 5 cm (p<0.001), absence of distant metastases at presentation (p=0.020), patients selected for curative surgical resection (p=0.002), and postoperative radiotherapy (p=0.003) were associated with prolonged survival time. CONCLUSION: In the selected patients(tumor size <5 cm, the absence of distant metastases at presentation, young age (<55), curative surgical resection and adjuvant radiotherapy may be associated with an increased survival time.