In an attempt to evaluate the diagnostic significance of the serum thyroglobulin (TG) in various thyroid disease states, authors measured serum TG by radioimmunoassay techniqe in 20 cases of normal subject, 22 cases of hyperthyroidism, 12 cases of diffuse nontoxic goiter (DNG) and 96 Gases of nodular nontoxic goiter (NNG). The results were as follows; 1) In 20 cases of normal subjects, serum TG level was 20.41±5.5 ng/ml (M±S.D.). There was no significant difference between males ans females. 2) In 22 cases of hyperthyroidism, serum TG level was 60.23±34.56 ng/ml and the range was from 22 to 175 ng/ml, which were significantly high levels comparing with normal controls (p〈0.01). 3) In 12 cases of euthyroidism with DNG, serum TG was 37.28±27.36 ng/ml and the range was from 14 to 89 ng/ml. In 96 cases of euthyroidism with NNG, serum TG was 70.43±78.18 ng/ml and the range was from 12.8 to 440 ng/ml. Both various showed significantly increased levels of TG than normal control (p〈0.01). 4) 57 cases of NNG patients were analysed pathologically by operation or needle biopsy and the TG level of each disease group is as follows. Thyroid carcinoma (l6 cases); 72.2±81.71 ng/ml, adenomatous goiter without cystic degeneration (15 cases); 74.86±45.64 ng/ml (M±S.D.) and adenomatous goiter with cystic degeneration (23 cases); 73.56±64.78 ng/ml (M±S.D.). There was no significant difference between each group. Also the TG levels of thyroiditis (5 cases) was 19.6±8.96 ng/ml (M±S.D.). 5) There were no significant correlations between serum thyroid hormones and serum TG in each thyroid functional states.