We treated 187 patients who had Graves' disease with low-dose radioactive iodide (131I), using a protocol that included a compensation for thyroid size. The incidence of early hypothyroidism (12 per cent) was acceptably low in the first year after 131I treatment, but we found a cumulative high incidence (up to 76 per cent) at the end of the 11th year. In contrast, the incidence of permanent hypothyroidism was relatively stable in 166 surgically treated patients, increasing from 19 to 27 per cent at the end of 11 years. Among 122 medically treated patients, only 40 per cent entered remission, and hypothyroidism developed in 2 per cent during the same period of follow-up. The long-term incidence of hypothyroidism in our patients treated with low-dose 131I therapy was much higher than that found in earlier studies using a comparable dose. Our study suggests that it will be difficult to modify therapy with 131I alone to produce both early control of thyrotoxicosis and a low incidence of hypothyroidism.