This study was desinged to evaluate the effect of radioactive iodine-131 therapy in differentiated thyroid carcinoma treated at the Presbyterian Medical Center, Chonju during the 20-year period from 1975 to 1994. The authors reviewed 246 patients who received radioactive iodine-131. An analysis of the therapeutic response and survival rates of the 246 patients has been carried out. The male to female ratio was 1 : 3.6. The peak incidence was in the 4th and 6th decades. The histologic findings in the 246 patients were papillary adenocarcinoma in 200 cases, follicular adenocarcinoma in 29 cases, mixed type in 14 cases, and others in 3 cases. Combined treatment modalities of 246 patients consisted of sugery and radioiodine in 222 cases, surgery with radioiodine and external irradiation in 11 cases, and surgery with radioiodine, external irradiation and chemotherapy in 5 cases. 42 of the 246 cases showed recurrence and the commonest type of combined treatment for recurrent case was surgery followed by radioiodine-131. The highest accumulated total dosage of radioiodine-131 was 480mCi in that case femoral metastasis was noticed. The most common locoregional metastatic site was ipsilateral cervical node, and neighbouring muscle, vessel, trachea, recurrent layngeal nerve, in order of frequency. The determinate 10-year survival rate was 91.8% in the group receiving surgery followed by radioiodine-13l and 71.4% in patients receiving surgery, radioiodine-13l with XRT. The determinate 10-year survival rate was better for patients under 40 years of age who received radioiodine as compared to patients over 40 year of age(85.7% vs. 33.3%). The most usual primary therapeutic dosage in the group of cervical lesion was 90$sim$120mCi after surgery.