During the course of twenty years 696 patients with hyperthyroidism and 690 cases of non-toxic goitre were treated with 131I in fractionated activities and controlled on an average 4,4 (1-18) years later. Treatment strategy included three special features: a. the first and any further activity amounted to not more than 1.85 MBq per g estimated thyroid weight; b. each therapeutic activity was accompanied by oral prednisone or prednisolone for 2-6 weeks; and c. after completion of radiotherapy each patient was put on thyroid medication which was not interrupted for purposes of control. Therefore, the rate of post-irradiation hypothyroidism could not be determined. All patients were found to be permanently euthyroid and none of the former hyperthyroid patients had relapsed. 84% of the hyperthyroid goitres and 78% of the non-toxic goitres had disappeared completely, 14% and 21%, respectively, were markedly reduced. Complete disappearance of the goitre was achieved with the first 131I activity in 35% of the patients with hyperthyroidism and in 48% of the non-toxic goitres, whereas 34% of the first and 35% of the second group required a second activity; the rest received three or more activities. The mean total activities of 131I necessary for complete reduction of the goitres depended on their size and amounted to 292, 507 and 1136 MBq, respectively, in euthyroid goitres with sizes, I, II and III. The corresponding figures in hyperthyroid goitres were 403, 577 and 1129 MBq, respectively. 314 patients had endocrine ophthalmopathy which was cured in 54% and significantly improved in 37%.