A 24-year-old woman presented with anaplastic transformation from papillary thyroid carcinoma with increased serum CA19-9. The patient had been diagnosed as having papillary thyroid carcinoma with lung metastasis at 11 years of age. She received a total thyroidectomy with cervical lymph node dissection followed by iodine-131 ((131)I) therapy over 12 years, but died due to sudden onset of rapid dissemination. Elevated serum CA19-9 was detected in the terminal stage, and anaplastic transformation was confirmed by post-mortem examination. Although there are few clinical reports suggesting a prognostic indicator for anaplastic thyroid carcinoma, CA19-9 may be a useful serum marker for this tumor.