Anaplastic thyroid carcinoma, which is one of the most aggressive tumors of the thyroid, has been seldomly reposted to have altered thyroid function. There have been few reports of patients with anaplastic thyroid cancer presenting in a hyperthyroid state. In the literatures, the mechanism of hyperthyroidism in anaplastic thyroid cancer is supposed that the rapid invasive growth of cancer seems to cause destruction of thyroid tissue and develops a hyperthyroid state, which is thought to be anlalogous to that of subacute thyroiditis and several types of metastatic cancer of the thyroid: rapid tissue necrosis with resultant release of thyroid hormone. Recently, we experienced a case of anaplastic thyroid cancer presenting with rapid growing mass and hyperthyroidism in a 67-year-old woman and report it with the review of the literatures.