Summary131I has been used as a treatment for hyperthyroidism in the last 40 years in over a million hyperthyroid patients. In subjects older than 40 year, the usual doses for the treatment of hyperthyroidism (<50 mCi) do not increase the risk for carcinomas of the thyroid or for other malignancies. Hypothyroidism is a very common side-effect, is dose related and increases at a rate of 3 % per year. Patients with a diffuse goiter or thyroid autoimmunity are at higher risk for developing hypothyroidism. If rapid correction of hyperthyroidism with high probability is needed, hypothyroidism is nearly inevitable. In such case substitution therapy has to be started early. Radiothyroiditis after treatment of131I for hyperthyroidism is relative rare. Pregnancy and lactation are absolute contraindications for treatment with131I. Genetic effects have not been observed in man. The lack of long-term observations after treatment with131I in children as well as the higher incidence of adenomas and carcinomas of the thyroid in such patients make the use of131I in children highly debatable.131I therapy is a safe, effective and simple therapy for hyperthyroidism in adults. In children and during reproductive years, however, its use is to be discouraged.