In contrast to the well-studied role of 5-lipoxygenase in the arachidonic acid cascade that occurs in inflammatory cells, the biological role of the related 15-lipoxygenases in the metabolism of free polyenoic fatty acids is far from clear. However, the activity of 15-lipoxygenases with more complex substrates may play a crucial role in the differentiation and maturation of certain cell types and in the oxidative modification of lipoproteins in the early stages of atherosclerosis.