Thirty-eight patients with metastatic well- differentiated thyroid carcinoma treated with 200mCi 131I were false negative serum thyroglobulin values during TSH suppression or at anti-thyroglobulin antibody(+) and discrepancies between findings of whole body scan and serum thyroglobulin level. After one to five cycles of 200mCi 131I therapy, complete remission and partial were achieved at 5.3% and 57.9%, respectively. We concluded that all of serum thyroglobulin, TSH, anti-thyroglobulin antibody, 131I or 123I whole body scan were necessary in follow up of metastatic well-differentiated thyroid carcinoma. Also, if there was no response after repetitive 200mCi 131I therapy, higher doses of 131I therapy should be considered.