Abstract. Thyroid function and thyroid gland volume, ultrasonically determined, were investigated in 27 hyperthyroid patients with solitary autonomous thyroid nodules before and during one year after 131I-treatment. Total thyroid volume decreased gradually from 40.9 ± 3.5 ml (mean ± sem) before treatment to 23.9 ± 1.8 ml (P < 0.001) at 3 months after 131I-treatment. No further change was observed. All but two patients received only one dose of 131I, and in spite of a significant decrease also of the non-adenoma side of the gland, none became hypothyroid. We conclude that 131I-therapy has an important place in the treatment of solitary autonomous thyroid nodules since all our patients became euthyroid within 3 months, only 2 of 27 patients needed more than one dose of 131I, no cases of hypothyroidism occurred, and thyroid volume was substantially decreased.