The highly unusual action of autoantibodies that stimulate TSH receptor is unique to the development of hyperthyroidism in Grave's disease. Assay of these antibodies has been used as a prognostic tool in monitoring response to antithyroid treatment, disease remission and relapse. The objective of this study was to evaluate the clinical value of TRab titre in terms of predicting responsiveness to antithyroid treatment by achieving a biochemical euthyroid status with minimum maintenance doses of antithyroid drugs. We followed up 28 patients (25 females, 3 males), 20 to 63 years old, who were newly diagnosed with Grave's disease. TSH receptor antibody (TRab) was measured prior to initiation of antithyroid medication. Based on TRab titres, patients were grouped into A (1.5-4.9 U/L), B(5-15 U/L) and C (>15 U/L). Our results showed that patients with lower levels of TRab (<5 U/L) before antithyroid drug therapy have statistically greater chance of achieving euthyroid status (33% in 6 months and 83.3% in 12 months) than those with higher TRab concentrations (combined B and C, 0% in 6 months and 18.8% in 12 months). This study suggests that TRab titre, detected by radioreceptor assay, is useful for predicting the outcome of antithyroid drug therapy in our patients with untreated Grave's hyperthyroidism.