Graves' disease is the most frequent cause of hyperthyroidism in the United States. It occurs predominantly in a genetically predisposed population. Autoimmune abnormalities constitute a major component of the disease. Immunoglobulins of patients with Graves' disease manifest a number of thyroid-targeted activities, including a thyroid-stimulating activity that is related to interaction of immunoglobulin G (IgG) with the TSH receptor. That hyperthyroidism in Graves' disease is caused by thyroid-stimulating IgGs is an attractive hypothesis but the evidence in favor of the hypothesis is still insufficient. A better alternative hypothesis is not available, however.