Background: There are three main methods of treatment in patients with differentiated thyroid cancer: surgery, 131I ablative therapy, and suppressive doses of thyroxine. The results depend on pathologic findings, clinical stage, and the extent of surgical resection. There is an inverse correlation between the volume of thyroid remnants and the effectiveness of radioiodine ablative treatment. The aim of the study was to estimate the effectiveness of 131I ablative treatment after late surgical radicalization in differentiated thyroid cancer patients who primarily had undergone non-total thyroidectomy. Material/Methods: An analysis was performed on 24 patients with nodular goiter who had primarily undergone less than total thyroidectomy and in whom post-surgical pathologic examination revealed differentiated thyroid cancer (19 patients with papillary and 5 with follicular thyroid cancer). Surgical excision was assessed as insufficient after ultrasonographic and scintigraphic thyroid remnant volume estimation. All patients were referred for surgical radicalization 6-10 weeks after primary surgery. An antero-posterior neck scintigraphy was always placed above the patient's head during surgery. Thirty-four thyroid remnants were removed and their real volumes (determined by immersing surgically removed thyroid remnants into a measuring cylinder filled with 0.9% NaCl solution) were compared with the volumes measured by ultrasonography and by scintigraphy. After 4-6 weeks from second surgery, the patients received 131I ablative therapy under endogenous TSH stimulation (58.7-99.1 ^IU/ml). The administered activity amounted to 2590-2812 MBq (70-76 mCi). Results: After incomplete primary surgery there was no significant difference in the volumes of thyroid remnants estimated by ultrasonography, scintigraphy, and the real volume read out from the measuring cylinder. 131I ablative therapy was effective in 95.8% of patients. During 3- to 8-year observation there was cancer recurrence in none of the patients. Conclusions: The values of ultrasound and planar 131I scintigraphy are similar with respect to the accuracy of volume estimation of thyroid remnants left after primary non-total thyroidectomy in differentiated thyroid cancer patients. Anterior-view planar 131I scintigraphy is essential for a precise location of thyroid remnants during surgical radicalization in patients with differentiated thyroid cancer. Late surgical radicalization effectively removes or significantly reduces thyroid remnant volume. After surgical radicalization, a very high efficacy of thyroid remnant ablation and a 3-8 year cancer recurrence-free period were achieved.