Isoprostanes are prostaglandin (PG)-like products of free-radical induced lipid peroxidation. Although the isoprostanes derived from arachidonic acid are the best characterized, many other polyunsaturated fatty acids can form isoprostanes. (±)5-iPF-VI is one of dozens of possible stereo- and regioisomeric isoprostanes which can be formed from arachidonic acid. To date, the most extensively studied of these is 8-isoprostane (8-epi-PGF, iPF-III). However, 8-isoprostane is a minor isoprostane constituent when compared to some of the other isomers which form in natural conditions of oxidative stress. (±)5-iPF-VI is an isoprostane from the unique Type VI class of isoprostanes. This class has been shown to be one of the major isoprostane products, in contrast to 8-isoprostane. In addition to being produced in greater abundance than 8-isoprostane, Type VI isoprostanes form internal lactones, which facilitates their extraction and purification from biological samples.