Background: Nitric oxide (NO) is a free oxygen radical studied in many tissues. Its tumour killing structure is shown especially by macrophages. The end products of NO are nitrite and nitrate. Their plasma levels are used biochemically to determine nitric oxide synthase (NOS) activity. The proliferative capacity of cancer cells accompanies the alteration in oxidant-anti-oxidant status. The risk of breast cancer is decreased in association with an increased level of polyunsaturated fatty acids in the erythrocyte membranes. The more the anti-oxidant capacity increases, the more the transformed cells grow. Malondialdehyde (MDA) is a lipid peroxidation marker, and low plasma levels of MDA are associated with advanced stages of breast cancer.