The Tc-99m-pertechnetate thyroid uptake rates(20 min) were measured in 24 healthy normal subjects, 140 patients with nontoxic goiter and 98 patients with thyrotoxicosis who were treated at the Thyroid Clinic, Seoul National University Hospital, from August 1972 to August 1973. Diagnostic reliabilities and correlations between 99mTcO4-thyroid uptake rate (20 min) and other thyroid function tests were evaluated. The observed results were as follows:1) The 99mTcO4- thyroid uptake rates (20 min) in normal subjects, euthyroid group and hyperthyroid group were 4.1±0.9%, 5.2±1.8% and 29.7±10.6%. There was a significant difference between the mean of the euthyroid group and the mean of the hyperthyroid group and so differentiation between them can be easy. 2) In the diagnosis of hyperthyroidism, the reliabilities of 99mTcO4- thyroid uptake rate(20 min), 131I thyroid uptake rate(24 hrs), serum T3 resin uptake rate, serum T4 and T7 were 87.9∼97.9%, 81.2∼94.4%, 87.9∼97.9%, 90.5∼99.3% and 93.7∼100%. 99mTcO4-thyroid uptake rate(20 min) is more accurate than 131I thyroid uptake rate (24 hrs) in the diagnosis of hyperthyroidism. 3) 99mTcO4-thyroid uptake rate (20 min) was well correlated with 131I thyroid uptake rate(24 hrs), serum T3 resin uptake rate, serum T4 and T7. Points in favor of Tc-99m are that it gives a small radiation dose to the thyroid, that tests can be repeated at the short interval, the study can be completed at a single patient visit and it is particularly well suited for the assessment of thyroid function in patients being treated with an antithyroid drug.