A clinical analysis was made on 161 cases of hyperthyroidism seen at the Radioisotope Laboratory of Kyungpook National University Hospital. This series consisted of 144 cases of diffuse goiter and 17 cases of nodular goiter. 1) Hyperthyroidism was most prevalent in the 4th decade and male to female ratio was 1:4.6. 2) Cardinal symptoms in the order of frequency were weakness, easy fatigability, palpitation, weight loss, nervousness, perspiration, heat intolerance, increased appetite, insomnia and dysmenorrhea. 3) Major physical findings in the order of frequency were goiter, fine tremor, tachycardia, wide pulse pressure, emaciation, warm moist skin, exophthalmos, systolic hypertension and atrial fibrillation. 4) The complications were ophthalmopathy (34.2%), thyrotoxic heart disease (5.6%), thyroid crisis (1 case), pretibial myxedema (1 case) and thyrotoxic myopathy (1 case). 5) Mean values of the six hour and twenty-four hour 131I uptakes by the thyroid glands were 67.5% and 71.6%, respectively, in diffuse goiter and 64.5% and 65.0%, respectively, in nodular goiter. 6) Mean values of twenty-four PB131I conversion ratio were 76.3% in diffuse goiter and 70.2% in nodular goiter and those of the basal metabolic rate was +51% in the former and +41% in the latter. Mean serum cholesterol level was 152mg% in diffuse goiter and that in nodular goiter was 175mg%. 7) Among the 134 cases treated with 131I, 66 cases (49.3%) were successfully controlled with single dose and in the majority of the cases the initial therapeutic dose required was 4.1∼4.0mC in diffuse goiter and 5.1∼6.0mC in nodular goiter. 8) With 131I treatment the symptoms improved in the following order:heat intolerance, emaciation, nervousness, insomnia, easy fatigability, weakness, fine tremor, goiter, perspiration, exertional dyspnea and palpitaion. And in a few cases improvement of even exophthalmos was seen. 9) Following 131I treatment myxedema occurred in 4 cases (3%) and reccurrence in 9 cases (6.8%).