The prevalence of toxic multinodular goiter (TMNG) is very rare in Japan which iodine intake is sufficient or excessive. It accounts for about < 1.0% of hyperthyroidism. The pathogenesis of TMNG is unknown, especially iodine rich area like in Japan although in iodine-deficient arears iodine insufficiency and TSH stimulating is the major promoting factors in its pathogenesis. Unlike Graves' disease, TMNG is more prevalent among aged patients and its symptoms of hyperthyroidism develops insidiously. Radionuclide imaging and ultrasonography provide very important information about the diagnosis of TMNG in addition to thyroid function tests. The treatments for TMNG are surgery after amelioration of thyroid function with antithyroid drugs, radioiodine treatment and PEIT (percutaneous ethanol injection therapy). We always have to pay attention to the existence of thyroid cancer complicated with TMNG.