A retrospective study was undertaken of 229 patients with hyperthyroidism due to Graves disease treated at the Cleveland Clinic with a large initial dose of sodium iodide I 131 (mean dose, 12.7 12.7±7.5 7.5 millicuries). The results indicate that a large initial dose is the most effective therapy for this disorder. Ninety percent of this group of patients required only one dose and recovered from hyperthyroidism within 12 weeks. Because we are convinced that there is no way to prevent the development of hypothyroidism in these patients, we have prescribed permanent thyroid hormone replacement after the hyperthyroidism has been controlled. In a mean five-year follow-up of 190 patients, 174 (91.5%) were still taking thyroid hormone.