Differentiated thyroid carcinoma accounts for about 85-95% of all cancers of the thyroid gland and the postoperative thyroid remnant ablation, which is performed by a wide variety of therapeutic activities 131I, is indicated in numerous patients. During the time period from 2004 to 2009, the research on a consecutive series of 259 patients with differentiated thyroid cancer was conducted. These patients underwent thyroid remnant ablation by different ablation activities of 131I, after total thyroidectomy had been preformed. In these cases the disease was confined to the thyroid bed, and no regional or distant metastases were confirmed by diagnostic tests during the course of the disease. The Kolmogorov - Smirnov test was used to show the distribution of values of certain variables, and in order of determining the differences in values of variables between the certain groups, the T-test and χ2 test were used. The ablation of residual thyroid tissue, in accordance with the applied activities, was evaluated as satisfactory. After the first application of 131I, the thyroid remnant ablation was significantly more effective in patients where high activities were applied 131I [3700 MBq (100 mCi)] than in cases where lower activities were applied [1110 – 1850 MBq (30 – 50 mCi) and 2775 MBq (75 mCi)]. There were no significant differences between the first application of 131I of low [1110 – 1850 MBq (30 – 50 mCi)] and intermediate [2775 MBq (75 mCi)] activities in terms of the success of thyroid remnant ablation. In comparison with the patients with multifocal tumors, the significantly more successful thyroid remnant ablation occurred in patients who had a tumor foci (unifocal tumors); the difference was clearly seen after the first application of 131I in the groups of [1110 – 1850 MBq (30 - 50mCi)] mCi and [2775 MBq (75 mCi)]. After the first application of 131I in the group of [2775 MBq (75 mCi)], and compared with the patients with multicentric tumors, the thyroid remnant ablation was more successful in patients with a tumor limited to a single thyroid lobe. Furthermore, it was concluded that, in most of the cases, it is reasonable that the low-activity 131I is applied, especially in patients with unifocal tumors. On the contrary, in patients with multifocal disease the application of high-activity 131I is justified. The application of intermediate activities and application of low activities were equally effective regarding the success of the thyroid remnant ablation.