Radioactive ioine(RAI), principally 131I, effectively controls hyperthyroidism in the majority of patients. The subsequent development of hypothyroidism, however, has been of increasing concern since it was first pointed out by Chapman and Maloof in 1955. And the steady increase of late hypothyroidism during the passage of time was known with its relation with dosage of RAI. The authors have investigated the development of hypothyroidism in 935 patients with diffuse toxic goiter(DTG) who were treated with RAI(131I) at the Seoul National University Hospital from 1960 to 1977 to reveal its relation with the number of RAI treatments, dosage of RAI, age of patients and exophthalmos with the following results. 1)The incidence of hypothyroidism by year after RAI therapy among 631 patients with DTG who were treated with single RAI regimen was 7.4%(1 year), 11.8%(2 year), 16.2%(3 year), 22.1%(4 year) and 25.5%(5 year), and that anong 163 patients given multipie RAI treatments was 8.6%(1 year), 10.4%(2 year), 13.3%(3 year), 29.1%(4 year), and 54.1%(5 year) respectively showing much higher yearly increments from 4 years after RAI treatmenet in comparison with the former. 2) Among 550 patients in the lower dose group treated with single RAI regimen less than 5.0mCi(Mean¡¾S.D.: 4.3¡¾0.6mCi), the incidence of hypothyroidism by year after RAI treatment was 6.8%(1 year), 11.4%(2 year), 15.4%(3 year), while among 81 patients in the higher dose group given single RAI treatment not less than 5.5mCi (Mean¡¾S.D.: 6.3¡¾0.5mCi) it was 12.0%(1 year), 15.4%(2 year) and 20.4%(3 year) respectively. However, the duration till euthyroid state after RAI therapy in the two groups was 5.1¡¾3.6 months and 4.8¡¾2.8 months respectively showing no statistically signficant difference (p>0.1). 4) The incidence of hypothyroidism after RAI treatment in patients younger than 30 years of age was 4.3%(1 year) and 7.7%(2 year); in patients from 30 years to 49 years of age, 5.8%(1 year) and 11.1%(2 year); and in those older than 50 years, 11.0%(1year) and 14.4%(2 year). The data revealed rising incidence of hypothyroidism with increase of patients' age. 4) Among 116 patients with exophthames the incidence of hypothyroidism by year after RAI treatment was 7.1%(1 year) and 12.15(2 year) while that among 184 patients without exophthalmes was 7.3%(1 year) and 12.2%(2 year) respectively. With the above data the authors could conclude that the hypothyroidism in patients with DTG who were treated by RAI developed more frequently than reported by others in Korea till now, and increased with the passage of time, the yearly increments from 4 years after RAI treatment increasing markedly in the multiple dose group, and the incidence could be reduced by decreasing the administered RAI dose not increasing the duration till euthyroid state after RAI therapy.