We report two cases who developed hyperthyroidism and positive thyroid stimulating antibody (TSAb) while they were treated with T[3] under the diagnosis of follicular adenoma of thyroid for four or eleven years, respectively. In both cases thyroid scintigraphy revealed an increased radioiodine uptake with multinodular defects. The uptake of 131I was not suppressed by the administration of T[3]. Histological examination showed encapsulated nodule surrounded by diffuse hyperplasia. TBII or TSAb appeared in accordance with increased plasma levels of T[3] and T[4]. In spite of suppressed TSH the appearance of TSAb and subsequent manifestations of Graves' disease suggested that T[3] failed to prevent the immune mechanism which was involved in the production of TSAb.