The benefit of I-131 therapy of metastatic differentiated thyroid carcinoma is controversial. To evaluate usefulness of I-131 therapy for metastatic differentiated carcinoma, 83 patients were chosen from 276 patients with differentiated thyroid carcinoma who were operated at Nagasaki University Hospital since 1960 according to the following criteria; (i) thyroids totally removed, (ii) existence of metastases at total thyroidectomy, (iii) the cause of death related to thyroid carcinoma if the patient died. The usefulness of I-131 therapy was evaluated by analysis of survival during 10 years following total thyroidectomy. In patients without remote metastases, the survivors and nonsurvivors were 37 and 2 in those without I-131 therapy (Group I), and 11 and 1 in those with I-131 therapy (Group II), respectively. There was no significant difference between the two groups. In patients with remote metastases, the survivors and non survivors were 9 and 1 in I-131 treated patients who had I-131 accumulation in the metastases with tracer doses (Group III-1), 1 and 11 in I-131 treated patients without I-131 accumulation in metastases with tracer doses (Group III-2), and 4 and 6 in non I-131 treated patients whose metastases were not examined for I-131 accumulation (Group IV), respectively. The prognosis was best in Group III-1 and followed by Group IV and Group III-2 in this order in patients with remote metastases. There was no significant difference in age and sex among the groups and in pathological findings of carcinoma. These results suggest that I-131 therapy may be useful for patients with remote metastases which accumulate I-131 with tracer doses but not for those patients who do not accumulate I-131 and that I-131 accumulation in metastases with tracer doses may be of prognostic significance.