From 1977 to 1982, 315 patients suffering from thyrotoxicosis with diffuse and/or nodular goitre or Graves' disease were treated with 131I. In 246 patients, the results of treatment after 7 months to 5 1/2 years could be evaluated. After a single treatment with 131I, 72% of the patients with hyperthyroid nodular goitre and 61% of those with diffuse goitre, but only 36% of the patients with Graves' disease showed normal thyroid function. By repeated 131I treatments--as many as 5 for those with Graves' disease--normal thyroid function could be achieved in nearly all patients examined. Patients with immune thyrotoxicosis, type Graves' disease, showed a high resistance to therapy. A dependence was found between the results of these treatments and previous antithyroidal drug therapy or surgery. The rate of hypothyroidism varied between 4 and 15%. The highest rate of hypothyroidism was observed in operated patients with persistent or recurring thyrotoxicosis.