A study of patients with recurrent thyrotoxicosis after subtotal thyroidectomy has shown that the operation has a profound effect on the natural history of Graves's disease. It is followed by pronounced changes in the immunological features of the disease, with a fall in the prevalence of serum thyroid autoantibodies, including the long-acting thyroid stimulator. Thyroid suppression returns to normal in 70% of patients. The treatment produces two populations of patients. In the larger group there is a permanent remission of the disease process. In the smaller group the disease process persists and, consequently, recurrent hyperthyroidism may develop. The mechanism of the change in the larger group of patients probably has an immunological basis.