Abstract. Serum thyroglobulin and goitre size were followed in 22 patients with simple goitre or single thyroid nodules during 9 months of thyroxine therapy, to see whether alterations in serum thyroglobulin correlated with changes in goitre size. In the case of such a correlation serum thyroglobulin could be used to predict which goitres respond to thyroxine therapy and which require surgery. Pretreatment serum thyroglobulin was elevated in 11 patients. It normalized in one of 7 patients whose goitre did not shrink and in none of the 4 patients whose goitre shrank during thyroxine treatment. Thus no simple correlation exists between alterations in serum thyroglobulin and goitre size during short-term thyroxine therapy. Since other studies suggest that increased serum thyroglobulin indicates ongoing goitre growth, thyroxine treatment might have been unsuccessful in all patients with persistently elevated serum thyroglobulin with a longer follow-up. The presence of predominantly thyroxine responsive tissue together with some autonomously growing, thyroglobulin-releasing areas in the same goitre could explain the failure of serum thyroglobulin to normalize in patients whose goitre shrank during therapy. The study shows that after eradication of iodine deficiency, thyroxine treatment is rarely successful in the Swiss goitre population and that surgical treatment is usually required.