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The thyroid uptake of 99m-technetium after 7 days of triiodothyronine was measured at the end of a course of antithyroid drugs in ninety-seven patients with thyrotoxicosis who were then followed up for at least 1 year. The relapse rate in sixty-six patients in whom the uptake was suppressed to less than 4% was 21% whereas it was 90% in those patients in whom the uptake was more than 4%. A prediction of the outcome on the basis of whether the uptake was more or less than 4% would have been correct in 82% of patients. Serial tests during treatment show that a correct prediction could have been made one year after starting treatment in 86% of cases depending on whether the uptake at that time suppressed to more or less than 6%. The initial unsuppressed 99m-technetium successfully predicted the likelihood of relapse after a course of antithyroid drugs in 75% of patients. We recommend that alternative therapy should be advised in those patients whose uptakes fail to suppress below 6% after 1 year of drug therapy.