Objectives: I-131 is a widely used radiopharmaceutical for the management of patients with differentiated thyroid carcinoma (DTC). It is the main agent for the ablation of residual thyroid tissue and the treatment of recurrent or metastatic DTC. However, diagnostic doses of I-131 larger than 3 mCi will cause some form of cell damage "stunning" which reduce subsequent uptake of the therapeutic dose of I-131. In contrast, I-123 is considered an ideal isotope for diagnostic studies, it emits only gamma radiation and thus lacks the stunning effect. The aim of the current study was to assess the impact of post-thyroidectomy I-123 versus I-131 Whole Body Scan (WBS) on the outcome of I-131 ablation for DTC. Methods: 70 consecutive DTC patients (60 papillary thyroid Ca., and 10 Follicular thyroid Ca.), underwent total thyroidectomy, followed by WBS, neck ultrasound (US), and subsequent I-131 ablation after 40 days without thyroid hormone replacement (TSH > 30 µIU/ml) were retrospectively enrolled in current study. Scans were acquired using the Forte dual head gamma camera (Phillips medical systems) equipped with a low energy parallel hole collimator, 24 hours after an oral administration of 37-111 MBq (1-3 mCi) of I-123, in 41 patients (Group 1), and equipped with a high energy parallel hole collimator, 48 hours after an oral administration of 74-185 MBq (2-5mCi) of I-131 in 29 patients (Group 2). 6 months later follow up I-131 WBS, neck US, serum thyroglobulin (Tg) and Tg antibodies (TgAb) were performed following suspension of L-thyroxin for one month (TSH > 30 µIU/ml) in 61 patients and following rhTSH stimulation in 9 patients. Results: No significant difference regarding age, gender and histopathology between both groups. 32/41, (78%) patients of group (1) and 16/29 (55%) of group (2) had negative follow up WBS, however, 5 patients of group (1), and 4 of group (2) with negative follow up WBS, had Tg > 5 ng/ml, with US findings suggestive of residual disease in the neck. The overall successful ablation rate were 27/41, 66% in group (1) versus 12/29, 41.4% in group (2) (P = 0.042). Conclusions: We found a significantly higher successful I-131 ablation rate among patients studied with I-123 WBS compared to those studied with I-131 WBS before radioiodine ablation, and recommend the use of I-123 instead of I-131 for WBS before radioiodine ablation.