Initial bone metastases in patients with differentiated thyroid carcinoma are rare, especially in younger patients. Long duration of therapy and high activities of radioiodine are often necessary to induce remission of metastatic disease. The curative potential of radioiodine therapy, in particular in younger patients, has not yet been determined. In this retrospective study we evaluated the therapeutic outcome, total radioiodine activities and associated side-effects in 107 patients with initial bone metastases. Eight of the 107 patients were younger than 45 (37.5+/-7.3) years, and were classified as group 1 (stage II, "low risk", WHO classification). The remaining 99 patients were older than 45 (64.1+/-9.5) years, and formed group 2 (stage IV, "high risk", WHO classification). Total or partial remission was more frequently achieved in group 1 than in group 2 (62.5% vs 49.5%). Lower activities were needed in group 1 (18.89+/-15.08 GBq vs 41.97+/-31.25 GBq), and there were less marked alterations in the blood count in this group. In group 1, blood count alterations reached only grade I or II (WHO classification), whereas grade III and grade IV alterations as well as acute leukaemia were observed in group 2. In group 1, complete remission was achieved with radioiodine therapy (11.1 GBq) in three out of four patients with < or =3 bone metastases. Additional pulmonary metastases (present in 44 out of 107 patients) did not influence prognosis. We conclude that initial bone metastases in differentiated thyroid carcinoma can be treated with curative intent by means of radioiodine therapy, and that this approach has a particularly realistic chance of success in younger patients and those with a small number of metastases.