Fifty seven patients with differentiated thyroid carcinoma were performed radioactive iodine- 131 whole body scans after administration of diagnostic dose (2∼10 mCi) and therapeutic dose (30∼150 mCi) within three months. We evaluated the state of radioactive iodine-131 uptakes in whole body scan to detect correct metastasis of thyroid carcinoma. The results are as follows: 1) In 20 of the 57 patients (35%), the therapeutic scan showed the additional uptakes that were not seen in the diagnostic scan. 2) In 9 (64.2%) of the 14 patients who had been received the thyroid ablation theraphy with I-131 previously, new additional lesions were found in the therapeutic scan but only 11 (25%) of the 32 patients who had not been received the thyroid ablation theraphy disclosed new uptake lesions (p〈0.01). 3) The additional uptake lesions of therapeutic scan were significantly more common in the bony metastatic foci (65.7%) than other areas (p 〈0.01). In 11 (55%) of 20 patients, additional uptake regions were anterior neck areas (thyroid bed or regional lymph node). We conclude that diagnostic scan with 2∼5 mCi I-131 is inadequate in evaluating residual iodine avid tissues of patients with thyroid carcinoma. Also post-theraphy I-131 whole body scan would be important to evaluate the correct staging and prognosis of thyroid carcinoma, and to follow-up patients.