The involvement of the immune system in the attainment and maintenance of carbimazole-induced remission in Graves' disease is poorly understood. We have investigated levels of T-lymphocytes expressing the HLA DR marker of activation and levels of thyrotropin receptor antibody (TRAb) in patients in sustained, carbimazole-induced, long-term remission from Graves' disease (median duration of remission 29 months, range 7-64). Levels of activated T-lymphocytes were increased in patients with long-term remission compared with controls (p less than 0.05) and were no different from the high levels found in patients with active Graves' disease. In contrast, levels of TRAb were normal in patients in long-term remission but high in patients with active disease (p less than 0.01). These results show that increased activated T-lymphocytes but normal levels of antibody to the thyrotropin receptor characterize patients in carbimazole-induced long-term remission of Graves' disease suggesting that activated T-lymphocytes may be important in the maintenance of remission.