Thirty (3.8%) of 780 patients with differentiated thyroid cancer seen between 1970 and 1987 had bone metastases. The primary tumor was follicular in 26 patients and papillary in four. Mean age at diagnosis was 61 years. The manifestation of bone metastases was the presenting symptom in 18 patients (60%). Treatment included total thyroidectomy, levothyroxine sodium therapy, and radioactive iodine treatments. Twenty-seven patients had bone metastases from the initial observation, with 44 sites involved. Of the sites, 27 (61%) were shown both on iodine 131 whole-body scan (WBS) and on x-ray film, 11 (25%) only on WBS, and six (14%) only on x-ray film. Multiple involvement was observed in 11 patients. The radiologic appearance was invariably osteolytic. Serum thyroglobulin was elevated in all patients. After radioactive iodine, no WBS+/X-ray+ metastases showed a complete response, although a sclerotic border was noted in several cases, whereas six WBS+/X-ray- lesions were no longer detectable by WBS. Treatment with radioactive iodine and bone surgery resulted in a complete cure in three patients and in a reduction of tumor mass in three. Twenty-one (70%) of the patients died of thyroid cancer after a mean survival of 86 months. Of the nine patients still alive, two are free of disease, three have a good quality of life, and four have severe disability.