A low iodine diet (LID) is the recommended preparation for radioactive iodine treatment. However, the recommended duration and stringency of LID are different among each recommendation. More stringent LID is expected in Korea because Korea is a iodine-rich region. We investigated the decrement of urine iodine excretion by two-week stringent LID for remnant thyroid ablation with radioactive iodine in Korean patients with thyroid cancer, prospectively. Material and Method: From November 2006, patients who referred to our hospital for remnant ablation after total thyroidectomy were included in this study. To decrease total body iodine, our protocol included three strategies. First, we checked medication which could inhibit the radioactive iodine uptake. Second, the date of I-131 treatment was scheduled at least 3 months later if contrast agent had been used. The last strategy was two-week stringent LID education by specialized nutritionist. Before and after two-week stringent LID, 24hr-urine iodine was analyzed respectively. 24hr-urine creatinine was also analyzed for determining more valid 24hr urine sampling subgroup. Results: Total 51 patients were finally enrolled. Average of 24hr-urine iodine excretion was significantly lowered (787±2242 → 85±85 μg/d, p=0.03) after LID and 74.4% of patients reached below the recommended urine iodine excretion level (<100 μg/d). In subgroup (n=14), similar results was showed (505±666 → 99±116 μg/d, p=0.05) and 78.6% of patients met the criteria. Conclusion: Most patients could reach below the recommended urine iodine level after two-week stringent LID. Therefore, in our opinion, at least two-week stringent LID should be recommended in Korea. (Nucl Med Mol Imaging 2008;42(5):375-382)