Thyrotrophin binding inhibiting immunoglobulins (TBII) were tested in 40 Graves' disease patients with hyperthyreosis, by an isotopic receptor assay (TRAK-Assay Behring). The samples were taken before, during and at the end of the hyperthyroid treatment. TRAb were present in serum of 86.67% of the patients before the treatment and in none of the 20 healthy control volunteers. There was no correlation between TRAb values and either the clinical signes of the disease neither the other thyroidal antibodies. After discontinuation of the antithyroid therapy (18 to 24 months after the onset) TRAb were negative in 71.4% of the patients; among this group 33% had a late relapse of the disease. In the positive TRAb group after discontinuation of the antithyroid therapy, 83.3% had a relapse of the disease and this relapse occurred shortly. There is a significant correlation between the TRAb values and the occurrence of relapse. In conclusion, TRAb values at the end of hyperthyroid treatment in Graves' disease patients seems to be a useful predictor value of the clinical course of the disease. This may be also a good help in the decision to continue or discontinue antithyroid therapy in Graves' disease patients with hyperthyreosis.