Serum thyroglobulin (Tg) was measured by radioimmunoassay in 81 patients with thyroid tumor who were treated in the department of Internal medicine of National Medical Center from January, 1981 to June, 1982. The results were as follows: 1) The mean serum thyroglobulin level in thyroid tumor was significantly higher than normal subjects (p〈0.001). 2) The mean serum thyroglobulin level in benign tumor was lower than malignant tumor (p〈0.05). 3) The thyroid carcinoma with metastasis had significant high level of serum thyroglobulin than without metastasis (p〈0.001). 4) The mean postoperative serum thyroglobulin level was significantly lower than preoperative level (p〈0.001). Data from our study show that serum thyroglobulin determination is useful for assessing the presence of malignant thyroid tumor, metastasis and the extent of residual or recurrent thyroid cancer after surgery.