Radioiodine (131I) treatment was started in Iceland in 1960 and the same formula has been used from the beginning to calculate the doses of radioactivity aiming for 70 Gy irradiation of the gland. In the present investigation we studied 468 patients who were treated over a period of 19 years (1973-1991). About 90% of the patients had Graves' disease (GD), 9% toxic adenoma but less than 1% toxic multinodular goiter. Approximately 70% of the GD patients became hypothyroid (subclinical hypothyroidism included) within the first year after a single radioiodine treatment and about 80% were hypothyroid four years after treatment with no significant increase after that. By contrast, only one of 15 patients with toxic adenoma became hypothyroid after a single treatment. For both groups the recurrence rate of hyperthyroidism was approximately 20%. The formula used for dose calculation in this study for GD patients does not seem to be satisfactory. The smaller glands are getting to much irradiation and the larger glands to little as can be seen by the frequency of hypothyroidism in the smaller glands and recurrences (continuing hyperthyroidism) in the larger glands after one treatment (table V). In 1993 blood samples were obtained from a sample group (n=103) of once 131I treated GD patients and measurements were done for serum TSH, T4 and free T4. One third of the patients who were considered euthyroid, and therefore not taking T4, were found to be hypothyroid with elevated TSH and low FT4 and one third of those taking T4 seemed to be overtreated with elevated FT4 and decreased TSH levels. It is concluded that the results of the radioiodine treatment for GD are unsatisfactory and need to be changed, either by adjusting the present regimen so that radiation is decreased in the smaller glands but increased in the larger ones or alternatively, by increasing the radiation dose to all the glands rendering the majority of the patients quickly hypothyroid followed by replacement therapy. The follow up of patients could be improved.