The present study was undertaken to investigate whether there is a rational basis for the usual long periods of thionamide therapy in patients with hyperthyroid Graves' disease. Eighty untreated patients were given the minimum dose of thionamide drug needed to maintain serum thyroxine, triiodothyronine, and thyrotropin (TSH) concentrations within their normal ranges. Thyrotropin-releasing hormone (TRH) tests were done at 6 monthly intervals for 2 years. Among patients who had positive responses of TSH to TRH, approximately 10 patients every 6 months were asked to stop thionamide therapy and were followed up for at least 1 year after discontinuation of drugs. In the groups treated for 6, 12, 18, and 24 months, relapses occurred in nine of 13, five of nine, three of 12, and two of 11 patients, respectively. Values for thyroid function tests before and at the end of treatment were not different among these four groups of patients. The overall remission rates were not ascertained. However, a minimum of 1 year's treatment is recommended, at least in Japan.