Purpose The purpose of this study is to compare posttherapy third day and seventh day I-131 whole body scans (3DWBS and 7DWBS) in detecting lung or bone metastasis from well-differentiated thyroid cancer. Materials and Methods We enrolled 52 patients with lung or bone metastasis out of 1,152 patients who were treated with high-dose I-131 therapy from January 2008 to June 2009. All patients underwent 3DWBS and 7DWBS. I-131 avidity was classified into three grades: no uptake, suspicious for uptake, and definite uptake. We compared the presence and grades of metastatic lesions on each scan. We categorized all cases into three groups based on I-131 uptake on each scan and compared several clinical parameters including FDG uptake and thyroglobulin (Tg) level among the groups. Results Sixty metastatic cases from 52 patients (45 lung and 15 bone metastases) were included. In 35 cases, I-131-avid metastatic lesions were detected by both 3DWBS and 7DWBS (group A). In 15 cases, metastatic lesions were missed on 3DWBS but detected on 7DWBS (group B). In 10 cases, I-131 uptake was not detected on either 3DWBS or 7DWBS (group C). Ten of 45 cases (22.2%) of lung metastasis that were negative on 3DWBS were detected on 7DWBS (p=0.002). Five of 15 cases (33.3%) of bone metastasis that were negative on 3DWBS were detected on 7DWBS (p=0.0625). The serum Tg level (TSH stimulated) was significantly different among groups A, B, and C (p=0.0030). However, after exclusion of cases without a history of I-131 therapy, there was no significant difference in serum Tg level among the groups (p=0.2330). The number of cases with a prior history of metastasis was higher in group A than in group B (p=0.0069). However, there was no significant difference in prior history of metastasis between groups A and C (p=0.8107). Conclusion 7DWBS showed more lung or bone metastatic lesions than 3DWBS. After high-dose I-131 therapy, 7DWBS should be considered regardless of the results of the 3DWBS for the diagnosis of lung or bone metastasis from well-differentiated thyroid cancer.