A follow-up study is presented in 88 patients treated with I-131 for Plummer's disease (localized autonomous thyroid function, either multifocal or as a solitary nodule) one to 17 years before the present study. Studies included clinical examination, scintigraphy, and function tests. One patient was hypothyroid, seven were marginally hyperthyroid, and five still received low dose antithyroid drugs. Of 75 euthyroid patients, the thyrotropin (TSH) response to thyrotropin-releasing hormone (TRH) was absent in 16 (generally with scintigraphic evidence of autonomous function), subnormal in 20, and normal in 39. A single autonomous nodule prior to treatment was found relatively frequently in males and in patients with a normal TRH test at follow-up. Most goiters had become smaller and one third of all solitary nodules could not be detected anymore. Autonomous function at follow-up was probably due to residual rather than recurrent disease in most, if not all, cases. It is concluded that I-131 therapy is at least as satisfactory as partial thyroidectomy in the treatment of Plummer's disease; lifelong follow-up was not found to be necessary.