The need for high dose radioiodine for ablation of remnants in patients with thyroid cancer is still in question. We compared the effectiveness of high and low dose 131I for ablation in patients in a prospective randomized study after surgical thyroidectomy. Twenty patients with differentiated pT2-3NoMo thyroid cancer were studied. The uptake was 5%-10% at 24 h. Ten patients received 100 mCi, the others 30 mCi 131I. Three months later all patients received a therapeutic dose of 150 mCi 131I. Another twenty patients with known distant metastases (pulmonary and/or bone) of differentiated thyroid cancer were studied. The remnant uptake was between 4%-10%. Ten patients received 300 mCi and ten 30 mCi 131I as ablation dose. Three months later all received 300 mCi 131I. The uptake at day seven was calculated for the same metastases from a whole body scan after both treatments. If effective ablation was defined as 24 h uptake in the remnant of less than 1%, then the ablation was effective in eight out of ten of the high dose and in seven out of ten of the low dose group. In pT2-3, N X M1 patients the ablation was effective in seven out of ten cases in both groups. If "effective" ablation was defined as an uptake of less than 0.5%, then the ablation was effective both in NoMo and in N X M1 patients in five out of ten with low dose and in six out of ten with high dose ablation treatment.(ABSTRACT TRUNCATED AT 250 WORDS)