Fifteen cases of Korean hemorrhagic fever who were admitted Chungnam National University Hospital from October 1981 to December 1981 were analysed on the evaluation of metabolic changes of the thyroid hormones, and thyroid function status in each clinical phase. 1) Serum T3, T4 concentration, FT4I and T4/TBG ratio were significantly lower (p〈0.001, p〈0.005, P 〈0.005, P〈0.001, respectively) than control group in oliguric and early diuretic phase of Korean hemorrhagic fever. With the recovery of illness, abnormal thyroid hormones and thyroid function indices tend to become normal range. But Serum FT4, TSH anlE TBG concentration were within normal range in all phase of KHF. Thus in Korean hemorrhagic fever, euthyroidism is probably maintained by normal or elevated serum FT4. 2) T4/T3 and rT3/T3 rato (p〈0.005, p〈0.005) were increased in oliguric and early diuretic phase of MHF. These results might be explained by decreased peripheral conversion of T4 to T3 in oliguric and early diuretic phase.