Hyperthyroidism is a hypermetabolic state accompanied by increased oxygen utilization, increased production of reactive oxygen species and consequentially measurable changes in antioxidative factors. Present study aimed at determining erythrocyte antioxidant enzyme activities and malondialdehyde levels of patients with hyperthyroid before and after treatment and comparing with controls. Therefore, the activities of whole blood glutathione peroxidase (GSH-Px), erythrocyte superoxide dismutase (SOD) and catalase (CAT) activities, serum malondialdehyde (MDA), total-T3, total-T4, free-T3, free-T4 and thyroid stimulating hormone (TSH) levels were determined before and after treatment in 22 hyperthyroid patients and in 32 healthy control subjects. Serum thyroid hormones and malondialdehyde (MDA) levels and erythrocyts superoxide dismutase (SOD) and catalase (CAT) were significantly increased, whereas glutathione peroxidase (GPx) activity was reduced in hyperthyroid patients compared to control. After propylthiouracil treatment, except catalase, all the parameters evaluated were normalized. Slight reduction in catalase levels after treatment was not significant compared to controls. Our findings show that incrased oxidative stres and decraesed antioxidant activity in the hyperthyroidism patients was prevented by a antithyroid drug like propylthiouracil.