Bone is the second most frequent target of distant metastases in patients with differentiated thyroid cancer, and such forms carry a very poor prognosis. The impact of 131I therapy in this setting is controversial. We describe the diagnostic circumstances and outcome of patients with bone metastases recently managed in two institutions. Among 921 consecutive thyroid cancer patients who had total thyroidectomy and 131I ablation between January 2000 and December 2004 and who were subsequently monitored, bone metastases had been diagnosed in 16 patients. In three cases, the bone metastases were non-functioning (negative 131I uptake) . These patients were treated with surgery and radiotherapy but progressed rapidly. The other 13 patients had functioning (positive 131I uptake) bone metastases. In five of them, thyroid cancer was revealed by signs of distant involvement (bone pain, n = 4; dyspnea, n = 1). The bone metastases progressed in these five patients, despite local therapy and multiple courses of 131I. The bone metastases in the remaining eight patients were discovered on the post-surgery 131I therapy scan. Complementary radiological studies were negative except in one patient in whom one of the metastases (a 5 mm lesion of the right humerus) was visible on magnetic resonance imaging (MRI). Six of these patients showed a good response to 131I therapy, with 131I uptake and Tg levels becoming undetectable or showing a sharp fall. One patient refused 131I therapy; bone metastases became visible on MRI within 1 year and the Tg level rose tenfold. The disease progressed in one patient despite 131I therapy. Post-surgical 131I ablation can contribute to early detection of bone metastases at a time when the Tg level may be only moderately elevated, when other radiological studies are negative, and when the disease is potentially curable by 131I therapy.