More than 500 sera were assayed for TBII under routine conditions using "Trak" assay in order to evaluate the sensitivity, specificity and prognostic interest of this determination in hyperthyroidism. The sensitivity for the diagnosis of Graves' disease was 83.5%, better in ophthalmopathic patients (93%) than in non ophthalmopathic patients (75%). The specificity was 99.4% with only one false positive in a hypothyroid patient. TBII level significantly decreases with carbimazole treatment except in patients who remain hyperthyroid. Determination of TBII before stopping carbimazole treatment or after surgery has a prognostic significance as a positive value indicates a relapse in almost all cases. Conversely, a fall of TBII to normal levels with treatment is insufficient to assess recovery. High levels are frequently observed after radioiodine therapy but do not indicate a poor prognosis.