A mixed differentiated thyroid carcinoma was found in a small asymptomatic nodule in a 44-yr-old woman with recurrent chest infections and bronchiectasis. After total thyroidectomy and 162 mCi (6 GBq) radioiodine ablation there was uptake in the thyroid remnant and in both lungs, interpreted as lung metastases. In 2 years she received further three 162 mCi (6 GBq) doses of 131I, as scans showed very similar lung activity. Another scan, during thyroxin suppression, showed again activity in the lungs. A 47-yr-old male patient with similar respiratory disease and no history of thyroid disorder volunteered to undergo radioiodine scan while on triiodothyronine suppression. His scan, too, showed concentration in the lungs. The female patient died 7 years after the diagnosis of lung thyroid metastases was made. No metastasis was found at autopsy. Radioiodine lung uptake may occur in patients with chronic inflammatory lung disease, presenting a potential diagnostic pitfall in patients with differentiated thyroid carcinoma.