Thyrotrophin binding inhibiting immunoglobulins (TBII) were measured in 27 patients with Graves' disease during and after longterm antithyroid treatment. The median observation period after treatment was 24 months. During the first 6 months of treatment the TBII index increased significantly in both the relapse and the remission group, but during the rest of the treatment and the observation no further change was observed. Patients with a TBII index below 0.35 at drug withdrawal (n = 8) all relapsed and patients with values above 1.00 (n = 5) all stayed in remission. There was a significant correlation between the TBII index at drug withdrawal and the time elapsed before a relapse. Graves' disease was treated with 131I in combination with carbimazole in 22 patients. The TBII index of these patients decreased after 131I and increased towards normal values during longterm observation of median 33 months. Of 4 patients with euthyroid ophthalmopathy one was TBII positive. This patient became overt hyperthyroid after an observation period of two years of prednisone treatment. It is concluded, that the TBII index is of some prognostic value after longterm antithyroid treatment, but is of no clinical importance following 131I treatment.