HYPERTHYROIDISM in identical twins is extremely rare. Reviewing the world literature in 1941, Bartels1found 10 authenticated sets. Boas, in an article on hereditary exophthalmic goiter,2cites 4 reported pairs of twins, only 1 pair being males. This is another report of exophthalmic goiter in male identical twins. One brother developed the disease at 19 years of age, and the other followed within a year. Both had severe neutropenia while in the hyperthyroid state, prior to antithyroid drug therapy, and both neutrophil counts increased after definitive treatment of the thyroid disease. The first was cured by subtotal thyroidectomy. His brother proved more difficult to prepare for surgery—his neutrophil level dropped on antithyroid therapy—and he eventually was treated with radioactive iodine. Report of Cases Case 1.— A 19-year-old Negro man had been well until December, 1954, when he began having nervousness, insomnia, profuse perspiration, tremor, loose stools, and