Differentiated thyroid carcinoma is an uncommon malignancy of childhood and adolescence that is unique because it has an overall favorable prognosis despite its relatively high rate of nodal and distant metastases. Total thyroidectomy and positive 131I therapy are recommended for cases with pulmonary metastases. In contrast, anaplastic thyroid cancer is one of the most aggressive malignancies that have an unfavorable and miserable prognosis. We report a case with an impressively long history. The patient had multiple pulmonary metastases that had been diagnosed by 131I administration when he was 14 years old, about 45 years before he underwent thyroidectomy. He had been kept unaware of his disease by his family and received no treatment for most of his life. Pulmonary nodules were noted at several medical checkups and showed a remarkable decrease in size during the untreated 44-year period after the 131I administration. At age 58, his thyroid cancer was first detected and total thyroidectomy was performed, with subsequent radioiodine therapy for pulmonary metastases. Unfortunately, anaplastic carcinoma developed and he died of disseminated tumors later.