Abstract： Objective To investigate the effect of iodine-131(131Ⅰ) therapy on patients with differentiated thyroid carcinoma concurrent metastases.Methods 50 cases with differentiated thyroid carcinoma after operation ac-cording to metastaticsites were given one of fractionated treatment,a total removal of dose(370～740)×107Bq,inter-val of 4 months.Results 35 of 50 patients(70.O%) had successful ablation of residual thyroid tissue after the first administration of radioiodine.Metastatic carcinoma in 18 cases,6 cases(33.3%) were cured,effective treatment was shown in 8 cases(44.4%)and treatment failure in 4 cases(22.2%);Thyroid metastasis before treatment were signifi-cantly higher than non-HTG thyroid metastasis(t=2.715,P<0.01),decreased significantly after treatment(t=2.844,all P<0.01);The contents of CD4+、CD8+、CD4+/CD8++ in treatment group after treatment[(26.5±4.7)%、(39.4±5.7)%、(0.6±0.4)%] were lower than before treatment[(38.3±5.6)%、(29.8±6.9)%、(1.4±0.5)%](t=2.345,t=2.244,t=2.451,all P<0.05).Conclusion Treatment with multiple hlsh doses of 131Ⅰ was safe and effective with little adverse side-effect.