Lung metastases were demonstrated by total-body 131I scans in 23 patients with differentiated thyroid carcinoma, at a time when chest x-ray was normal. This total-body 131I scan was performed after the administration of 2 mCi (in 11 patients) or 100 mCi (in 12 patients). Overall uptake of 131I in lungs was less than 1% of the administered dose in 11 patients. All patients were treated with radioiodine. No lung uptake was found in 20 patients at the last 100 mCi post-therapy scan. Among them, Tg level became undetectable during T4 treatment in eight, lung CT scan showed the disappearance of the micronodules in seven, and lung biopsy did not show evidence of disease in two patients. No patient developed radiation lung fibrosis. In conclusion, favorable responses to radioiodine treatment were observed despite relatively low overall uptake, in relation to the small size of lung metastases. This provides high concentrations of radioiodine and therefore high radiation doses.