Thyrotropin (TSH)-receptor autoantibodies were assessed by measurement of 125I-bTSH binding inhibition in 32 patients with juvenile-onset Graves' disease (one of whom was studied twice) and 16 normal control subjects. Thirteen (76.5%) of 17 thyrotoxic patients had significantly elevated TSH binding-inhibitory immunoglobulin (TBII) activity compared with eight (50%) of 16 patients who were in clinical remission and none of the control subjects. Mean TBII activity was significantly greater in thyrotoxic patients than in individuals in remission, except in one unusual patient in whom there was a discordance between TBII activity and in vitro thyroid-stimulatory activity. In eight euthyroid patients who were followed up for at least five months while not receiving treatment, assessment of TBII activity did not predict who would or would not suffer relapse at a later date. Thus, TBIIs are secreted in excess in juvenile Graves' disease, the titer decreasing as the disease remits. The TBII assay cannot be used as the sole predictor of when antithyroid medication can be withdrawn safely, however.