Purpose: We compared the first postoperative diagnostic and post-therapy scans of patients who received therapeutic doses of I-131, to investigate the difference in clinical outcomes between patients with concordant findings of diagnostic and post-therapy scans and patients with discrepant (more lesions in post-therapy scan) findings. Materials and Methods: The first postoperative diagnostic and post-therapy radioiodine scans of one hundred forty three patients with well differentiated thyroid carcinoma were reviewed. Diagnostic scans were obtained following ingestion of 185 MBq of I-131 and post-therapy scans were obtained after therapeutic dose of 3.7∼9.3 GBq of I-131. Successful ablation was defined as no radioiodine uptake on diagnostic radioiodine scan and normal range of serum thyroglobulin level (<10 ng/ml) during serum TSH elevation. Results: Discrepant scan findings were noted in 25 (17.5%) patients. Twenty-two patients (15.4%) showed more lesions in post-therapy scan and 3 patients (2.1%) showed stunning effect. Nine (64.3%) of 14 patients with distant metastasis revealed metastatic lesion(s) only on post-therapy scan. Stunning effect was considered as sublethal damage in 1 patient and treatment by a diagnostic dose in 2 patients. Ablation was achieved in 52.4% (75/143) of all patients. Ablation rate and mean cumulative radioiodine dose were not different statistically between concordant and discrepant groups. Conclusion: There were 17.5% difference between diagnostic and post-therapy scan findings when using 185 MBq of I-131 as a diagnostic dose. However, 64.3% of distant metastases were revealed only on post-therapy scan. Ablation rate and mean cumulative radioiodine dose were not different statistically between concordant and discrepant groups. The stunning effect was considered as not only sublethal damage but also treatment by a small diagnostic dose of radioiodine. (Korean J Nucl Med 2000;34: 22-9)