We studied the effects of radioiodine therapy (RIT) for autonomously functioning thyroid nodules (AFTNs) and Graves' disease on thyroid function and size up to one year after RIT. In 230 patients with AFTNs, a dose of 300 Gy was effective in about 90% of the cases 6 months after RIT. Out of 65 patients suffering from Graves' disease, 5 patients (8%) had persisting hyperthyroidism 6 months after RIT with a dose of 150 Gy. This group consisted exclusively of patients with manifest hyperthyroidism at the time of RIT. As determined by ultrasonography 6 months after RIT, a reduction of thyroid size by about 40% and 60% was observed in patients with AFTNs and Graves' disease, respectively.