To clarify the hematologic effects of the radioiodine ($^{131}I$) in therapeutic doses ($5{sim}10$ mCi) on the various thyroid patients, authors studied the peripheral blood pictures of 396 goitrous patients before and after radioiodine ($^{131}I$) administrations in the Isotope Clinic of Seoul National University Hospital. Among these 396 cases of goiters, we gave 5 to 10 mCi of radioiodine ($^{131}I$) with single or fractionated administrations. The blood pictures of peripheral blood were repeated after 3 months in 40 cases of 65 cases who had been treated with $^{131}I$. The blood pictures of non-treated thyroid patients were compared with that of normal Korean values to clarify any difference between normal and goiter. The blood pictures of hyperthyroid patients treated with $^{131}I$ therapy were compared with the blood pictures of non-treated thyroid patients. The results were as following: 1) The incidence according to type: Toxic diffuse goiter: 35.4% Nontoxic nodular goiter: 29.7% Euthyroid: 13.8% Nontoxic diffuse goiter: 12.6% Hypothyroidism: 4.3% Thyroiditis($ar{s}$ subacute form): 1.8% Toxic nodular goiter: 1.4% Malignancy: 1.0% 2) Age incidence: The range of distribution was 11 to 71 years. The peak incidence was found in the 4th decade of life. $80.6{sim}82.6%$ of those 396 cases were found among the 3rd, 4th and the 5th decades of life. 3) Sex incidence: Sex ratio of male:female was 1:7.8. 4) The most outstanding findings in peripheral blood before treatment were decreased erythrocyte count and hemoglobin value in all types of thyroid diseases, especially in. the cases of hypothyroidism and thyroiditis. Hook worm-infested patients showed no significant difference in erythrocytes and hemoglobin values from those of other hook worm free patients. 5) Total leukocytes count was within normal range. Differential count of W.B.C. showed increased percentile of lymphocyte in diffuse toxic goiter and thyroiditis. 6) 39 cases of diffuse goiter treated with $^{131}I$ toxic showed amelioration in the anemia and restoration to normal range of lymphocyte count in association with increased percentile of neutrophiles 3 months after administration, except a case of toxic nodular goiter. One can observe anemia in slight degree, and increased lymphocytes count in hypothyroidism. Therapeutic dose of radioiodine ($^{131}I$) does not result any residual effect on the hematopoietic function. Radioiodine ($^{131}I$) therapy resulted in improvement of thyroid function in association of amelioration of pevious abnormal blood pictures. 7) Authors did not observe any myxedema resulted from radioiodine therapy during the 3 months period in this study.