Between 1977 and 1984 a total of 301 patients with autonomous thyroid adenoma were irradiated with an individually calculated one-time dose of 400 Gy units of 131I. Repeated follow-up tests were made in 217 patients (up to 7.2 years, mean of two years). Pre- and post-treatment diagnosis in all patients consisted of determining T3 and T4, one TRH test, one 131I two-phase test to determine treatment, including quantified scintigraphy (under suppression, if necessary), as well as post-treatment scintigraphy and (post-treatment) 99mTc scintigraphy. The treatment was successful in 98% of patients; there was no difference between compensated and decompensated forms. Euthyroid state was achieved in 87% of patients, with typical findings of compensated T3 oversecretion, as is known to occur with endemic goiter in regions of iodine deficiency. The ability of the thyroid for autoregulatory adaptation to such iodine deficiency is thus preserved. Preclinical hypothyroidism occurred in 11% of patients: it could have been avoided in about half of them. Persistent or recurring autonomous adenoma was observed in 2% of patients as a result of under-dosage. One should thus aim at a dose of 400 Gy, to obtain optimal elimination. Radiation-induced carcinogenesis was not observed: radioiodine treatment and operation are thus of equal value in the causal treatment of autonomous adenoma. Radioiodine treatment is indicated in patients aged over 40 years with additional diseases and increased risk of anaesthesia and operation. It is preferred treatment if there are multiple autonomous adenomas.