Oxidant/antioxidant balance has been suggested as an important factor for initiation and progression of cancer. The objective of this study was to determine changes in the levels of malondialdehyde (MDA), nitric oxide (NO), total cholesterol, triglycerides, LDL-cholesterol, HDL-cholesterol, total antioxidant capacity (TAC), glutathione peroxidase (GSH-Px), and superoxide dismutase (SOD) activities in serum samples of breast cancer patients (n=30) and healthy subjects (n=100). MDA and NO levels were found to be increased in breast cancer patients compared to the healthy subject group (p<0.05). Total cholesterol and triglycerides were elevated; and HDL-cholesterol level was found to be decreased in the cancer patients as compared to the healthy subjects (p<0.05). Compared to the healthy group, both serum TAC levels (p<0.001) and activity of SOD and GSH-Px (p=0.05) were found to be decreased in the breast cancer patients as compared to the healthy controls. Considering the data presented in this study, we suggest that free radicals induce lipid eroxidation and peroxidation of unsaturated fatty acid with decreased activity of enzymatic antioxidants in breast cancer.