To evaluate the significance of assay of serum thyroglobulin(Tg) in monitoring the course of the thyroid cancer or its response to treatment, serum thyroglobulin levels were measured in 41 patients with thyroid cancer who visited Seoul National University Hospital from August, 1981 to August, 1982. The results were as follows: 1) Serum Tg levels $1sim3$ months after thyroidectomy was $31{pm}23$ ng/ml$(mean{pm}S.D.)$ in 14 patients without metastasis, $66{pm}41$ ng/ml in 21 patients with regional metastasis and $176{pm}59$ ng/ml in 6 patients with distant metastasis and there were significant differences among three groups(p<0.01). 2) Serum Tg levels in 13 patients with metastasis before and after $^{131}I$ treatment were $134{pm}62ng/ml$ and $67{pm}52ng/ml$ respectively. 3) In the follow-up measurement of serum Tg levels every 3 months for about 1 year, almost all serum Tg levels were below 60 ng/ml in 12 patients without distant metastasis and serum Tg levels were elevated above 60 ng/ml in 5 of 6 patients with distant metastasis. 4) In 6 patients with distant metastasis, serum Tg levels were elevated in 5 patients and $^{131}I$ whole body scan showed definite metastatic evidence in 3 patients and suspicious evidence in 1 patient. From above results, we concluded that serum Tg level is very useful as an indicator of recurrence or metastasis in patients with thyroid cancer after operation.