Indications for complementary radioiodine therapy of differentiated thyroid carcinoma are based on its potential to sterilize micrometastases and to ablate thyroid remnants. A short summary of the assumptions leading to the combination of total thyroidectomy with complementary radioiodine treatment is given in the paper. We also present the preliminary results of the evaluation of nearly 800 patients with differentiated thyroid carcinoma treated surgically in various centres and referred to our department for radioiodine therapy. Total 10-year survival was significantly higher in patients in whom complementary radioiodine therapy had been introduced after total or less than total thyroidectomy. The upper limit of postoperative 24 h thyroid uptake of 20% before radioiodine therapy was revealed as a good criterium for the optimal long-term effects. However, a gradual decrease of postoperative 131I uptake has been observed, thus, this upper limit is probably to be lowered in future.