Activating mutations of TSH-R have been described in toxic nodules and more recently in familial nonautoimmune thyrotoxicosis. This last entity is still confused with familial Graves' disease and the aim of this study is to define its phenotype. Based on 49 patients coming from our first family and on the 4 other kindreds secondarily described in the literature, the phenotypic expression is: a high incidence of hyperthyroidism, an early onset of disease, a higher men/women ratio (17/32) than in Graves, disease, the absence of ophthalmopathy and of circulating and intrathyroid signs of immunity, a pathology similar to toxic nodule, the need for a total destruction of thyroid tissue to cure the patients. The total analogy with toxic nodule leads us to name this new entity "toxic hyperplasia'. Among 92 successive diffuse nonfamilial thyrotoxicosis cases (initially considered as Graves) we isolated 5 cases without extra- and intrathyroidal autoimmunity, raising the question of the existence of an apparent "sporadic' form of toxic hyperplasia (neomutation?).