Serum TBII measured by radioreceptor assay using 125I- bovine TSH and porcine thyroid well membrane was checked before, 6 months and 12 months after initiation of thionamide regimens in 63 Graves' disease patients and was related with their remission state. 1) A significant difference (p〈0.01) in pretreatment TBII was noted between the remitted [N=45, TBII 40.9±18.2% (mean±S.D)] and the unremitted (N=18, TBII 64.1±15.3%) groups. 2) After 6 months of therapy, TBII were significantly decreased in both groups (to 20.2±10.3% and to 45.2±16.3%, p〈0.05 for each group). 3) At 12th month, TBII activities were not significantly decreased compared to the 6th month levels in both groups. 4) 3 of the 58 patients who were initially TBII positive (over 15%) converted negative. All the 3 belonged to the remitted group. 5) No significant differences were seen in initial and posttreatment TBII levels between propylthiouracil treated (N=36) and methimazole treated (N=27) cases. with above mentioned results, we observed that the TBII decreased significantly with 6 months of thionamide therapy and concluded that the pretreatment measurement of serum TBII may be clinically useful in predicting the response to thionamide regimen in the treatment of Graves' disease.