Between 1963 and 1971, radioiodine (133I) treatment of a toxic thyroid adenoma was performed in 229 patients, in 70% the dosage being 30000 rd. In 188 patients the results were checked an average of 2 1/2 years later. Subjective improvement or freedom from symptoms was achieved in 76% (in 85% within 3-6 months), body weight increased, serum cholesterol level rose significantly, while pulse rate, pulse amplitude, PB127I and index of free thyroid hormones significantly fell. The 131I two-phase test was inconclusive. 72% of toxic adenomas were inactivated and 88% of patients with still active adenomas were improved or free from symptoms. Clinical improvement apparently preceded the scanning results. Corresponding to the index of free thyroid hormones, 94% were euthyroid, 0.6% hyperthroid, and 2.8% suspected of being hypothryoid. In combination with other findings, especially isotope scanning, 1.7% of patients were suspected of having T3 hyperthyroidism after treatment. A recurrence was observed in 2.1%. The success rate increased with rising radiation dose up to 30000 rd.