Background: Breast cancer is caused by breast tissue malignant cells and it has become one of the main medical concerns with a socio-economic significance especially for women. Among the multiple factors involved in the initiation, progression, and invasion of breast cancer, oxidative stress plays an important role. Antioxidant status, lipid peroxidation, and oxidative stress in newly diagnosed breast cancer patients were determined to find a defined pattern of oxidative stress in these patients. Methods: The malondialdehyde (MDA) levels (as an indicator of lipid peroxidation), glutathione peroxidase (GPX), and superoxide dismutase (SOD) activities of newly diagnosed breast cancer patients (n=38) and controls (n=38) were assessed using blood samples. Results: MDA level and SOD activity were significantly higher in the breast cancer patients compared to the healthy subjects group (p<0.05). Compared to the healthy group, GPX activity decreased significantly in patients group (p<0.05). Conclusions: High lipid peroxidation is an important risk factor for breast cancer and the increased levels of superoxide anion in breast cancer cells may be a reason for the induction of SOD activity. Nevertheless, oxidative stress is an important factor in development and progression of breast cancer. Further studies on it can lead to a more helpful approach to management of breast cancer.