About 20% of breast cancer patients over-express the human epidermal growth factor receptor-2 (HER2), which is associated with enhanced tumor malignancy. The influence of HER2 overexpression on oxidant/antioxidant parameters in humans remains unknown; therefore, we investigated the oxidative profile in women according to their HER2 status. Fifty-two controls and 52 breast cancer (BC) patients were enrolled. The BC patients were subdivided into HER-, negative for HER2 overexpression, and HER+, positive for HER2 overexpression. Oxidative stress profilling was measured by malondialdehyde (MDA), free 8-isoprostane F2, protein carbonyl content, nitric oxide (NO), total radical antioxidant parameter (TRAP), superoxide dismutase (SOD), catalase activity, and glutathione (GSH) levels. Total thiol content and lipoperoxidation were evaluated in HCC1954 and MCF-7. Cells overexpressing HER2 presented enhanced oxidative stress. Increased erythrocyte lipoperoxidation was found in BC patients, while plasma lipoperoxidation was detected in both the BC and HER- groups. Decreased MDA levels were found in the HER+ group, suggesting that HER2 overexpression may protects against plasma lipoperoxidation. No alteration was found for 8-isoprostane F2, NO, and carbonyl content. TRAP was decreased in BC patients, while HER2 overexpression increased SOD and prevented decreased GSH levels. These data help to understand the HER2 overexpression in oxidative signaling and may enable the development of new strategies for anti-HER2 therapy.