Background and Aim : Hyperthyr-oidism is a hypermetabolic state associated with increased oxygen consumption that may increase free radical generation, therefore oxidative stress may play a role in the patho-genesis of tissue damage caused by hyperthyroidism. The aim of the present study was to evaluate oxi-dant-antioxidant status in patients with hyperthyroidism and the effects of antithyroid treatment. Methods : We studied 20 untreated newly diagnosed hyperthyroid patients (group 1, 14 women and 6 men, 33.2±7.8 years), 16 treated hyperthyroid patients who achieved euthyroidism with antithyroid drug (carbimazole) (group II, 11 women and 5 men, 34.2±9.5 years) and 20 age and sex matched healthy control subjects. Recorded data included weight, BMI, heart rate, blood pressure, and lipogram. Malon-dialdehyde (MDA), nitric oxide (NO), reduced glutathtone (GSH), catalase (CAT) and superoxide dismutase (SOD) were assayed in all patients and controls. Results: Compared with control group, group I patients had an increase in MDA (P<0.01), and NO (P<0.01) and a decrease in GSH (P<0.01), CAT (P<0.05) and SOD (P<0.01). Compared with group II. group I patients had increased MDA and NO (P<0.01) and decreased GSH (P<0.01). CAT (P<0.05) and SOD (P<0.01). Group II patients had increased MDA in comparison to control group (p < 0.05), however, NO, GSH, CAT and SOD did not show significant differences between the