It was in 1942 that radioactive iodine, I-131, was applied to therapy of Graves' disease. Recently, this therapy is popular in Japan because the maximum therapeutic dose of I-131 for outpatients was estimated. The feature of I-131 therapy in patients with hyperthyroidism due to Graves' disease is as follow as; 1. The most effective method of I-131 therapy is performed under the state of hyperthyroidism. 2. The symptom of hyperthyroidism will be disappeared from 1 month after the therapy gradually. 3. Transient hypothyroidism may be appeared 3-4 months after the therapy. 4. Hypothyroidism after I-131 therapy will be difficult to avoid in their future. I-131 therapy should be undertaken in the patients with uncontrolled hyperthyroidism by medication of anti-thyroid drug.