Lipid peroxidation may play a very important role in cell proliferation especially those of tumors. Secondary products of lipid peroxidation may interact in an inhibitory manner with various cell processes and/or cycle phases that are essential for cell division resulting in a decreased tumor growth rate by killing actively dividing cells of the growth fraction and probably increasing cell loss. The inhibitory or static action of diets containing elevated levels of fish oil on tumor growth may be via lipid peroxidation control over cell proliferation.