Over the past 6 years, from May 1960 to June 1966, 1,716 patients with various diseases of thyroid were examined and thyroid function tests with 131I were done. Among them, 545 patients with hyperthyroidism were treated with 131I. A summary of the clinical data of the 131I-thyroid function tests and the therapeutic results of 131I were presented and discussed. 1) The patients examined consisted of;596 cases (34.7%) with toxic diffuse goiter, 412 cases (24.0%) with non-toxic nodular goiter, 278 cases (16.2%) with euthyroidism, 236 cases (13.8%) with non-toxic diffuse goiter, 89 cases (5.2%) with hypothyroidism, 53 cases (3.1%) with toxic nodular goiter, 32 cases (1.9%) with thyroiditis and 20 cases (1.2%) with dyshormonogenesis. 2) There were 218 (12.7%) male patients and 1,498 (87.3%) female patients, showing a ratio of 1:6.9, female predominantly. 3) The majority of patients (79.6%) were in the 3rd through 5th decades of their lives showing the peak in the 4th decades (35.9%). 4) The diagnostic values and normal ranges of 131I uptake test, 48 hour serum activity, T3 red blood cell uptake and PB131I conversion ratio were discussed. 5) An attention was given to dyshormonogenesis, a qualitative hypothyroidism, due to its characteristic findings of clinical and 131I thyroid function tests, and its pathogenesis was briefly reviewed. 6) Among 545 patients with hyperthyroidism treated with 131I, 68.3% was cured after single therapeutic dose and another 24.0% was cured after second dose. 7) The complications of 131I therapy were discussed in some details and myxedema had developed in 3.9% of our cases. No thyroid cancer was found after 131I therapy.