Klinefelter's syndrome is one of the most common forms of primary hypogonadism and infertility in males. It is characterized by small and firm testes, gynecomastia, azoospermia, and an elevated gonadotropin level. The frequencies of diabetes mellitus, breast cancer, and germ cell neoplasia increases in Klinefelter's syndrome. We report upon a 35 year-old male patient with Graves' disease in association with Klinefelter's syndrome; as confirmed by chromosome analysis. The patient is being treated with antithyroid medication for Graves' disease and by testosterone replacement for Klinefelter's syndrome.