Radioactive iodine treatment is currently the most popular treatment modality for hyperthyroidism in many patients of the world. In Japan, however, a considerable number of patients with hyperthyroidism have undergone successful surgery. To elucidate the advantages and disadvantages of surgical and 131I treatment, the thyroid function of 66 patients from each group was compared, 5 to 8 years after treatment. None of the surgically treated patients had been reoperated upon on, whereas 12 (18 per cent) of the patients treated with 131I required 2 or more doses. The number of patients with normal ranges of serum T3, T4 and TSH values was almost identical: 33 (50 per cent) of the postoperative patients and 31 (47 per cent) of the post-irradiated patients. In the surgically treated group, there was no tendency for hypothyroidism to develop with the passage of the time. However, although the distribution of T3 and T4 in the normal control and postoperative groups showed a normal bell-shaped distribution, the post-irradiated group did not exhibit this distribution. The normal control group and the postoperative group showed positive correlations of T3 and T4 but the post-irradiated group did not (p less than 0.05). When the titers of antithyroglobulin and antimicrosomal antibodies prior to therapy were compared with those following therapy, they had decreased in 41 per cent and 76.8 per cent, respectively, of the postoperative cases, but in only 24.2 per cent and 45.5 per cent of the post-irradiated cases.