A 75-year-old man with thyroid tumor that had metastasized to the lung was admitted to this institution in a condition of cardiac failure. Aspiration biopsy of the thyroid mass showed hypoplastic papillary carcinoma, and a diagnosis of Graves'disease was made through serum-function studies and titers of TBII and TSAb during the course of hospitalization. The patient refused to undergo thyroidectomy, but ultrasonography was used to observe the lesion on six occasions. Volume, estimated using the formula for a rotated ellipse, was 16.7 cm³ on hospital days 1 and 71, 66.6 cm³ on day 167, 112.9 cm³ on day 188, 128.7 cm³ on day 195, and 219.8 cm³ on day 203. Ultrasonographic findings suggested a diagnosis of papillary carcinoma of the thyroid on days 1 and 71, but on day 167 the shape of the tumor began to change from that of an irregular star to one resembling a clover-leaf; its final shape was round. The source of multiple small, strong echoes from the center of the tumor on day 1 had gradually shifted to the margin of the tumor by day 167. On day 203, aspiration biopsy of the thyroid confirmed the diagnosis of anaplastic carcinoma. These observations suggest that rapid growth and change in shape of an irregular thyroid nodule may be characteristic ultrasonographic findings indicative of anaplastic transformation in thyroid cancer.