Many textbooks claim that radioiodine (131I) treatment should be avoided in treatment of a goitre with substernal extension, due to fear of acute swelling of the gland with resulting respiratory problems. We examined patients with multinodular goitre, either nontoxic (n = 20) or toxic (n = 10) after treatment with 131I. An ultrasonically determined thyroid volume and thyroid function variables were investigated before and two, seven, 14, 21, 28 and 35 days after treatment. In nontoxic goitres the thyroid volume did not increase significantly, the maximum increase in the median volume being 4% on day 7. Serum levels of free T3 and free T4 indices increased by 20% (day 7) and 13% (day 14) (p = 0.002), respectively. Likewise thyroid volume in toxic nodular goitre did not change significantly after 131I treatment. None of the patients presented symptoms of tracheal compression. We conclude that 131I treatment of nontoxic as well as toxic multinodular goitre does not seem to increase thyroid volume.