Isoprostanes are members of a complex family of lipids, isomers of the conventional enzymatically derived prostaglandins (PG), which are produced in vivo primarily, if not exclusively, by a free radical-catalyzed peroxidation of polyunsaturated fatty acids. Most of the work has been focused upon a group of isomers of the enzyme-derived PGF(2alpha), called F(2)-isoprostanes (F(2)-iPs). Because of their mechanism of formation, chemical stability and the rapid development of sensitive methods for their measurement, they have the attraction as non-invasive indices of oxidant stress in vivo. Altered generation of F(2)-iPs has been reported in a variety of clinical settings putatively associated with oxidant stress. These include atherosclerosis, chronic obstructive pulmonary disease and Alzheimer's disease. Furthermore, the measurement of specific F(2)-iPs may provide a sensitive biochemical basis for rational dose-selection of natural and synthetic inhibitor of lipid peroxidation. Although F(2)-iPs possess biological activities in vitro and in vivo, much remains to be learned about their role and as mediators of the cellular effects of lipid peroxidation and oxidant stress.