Diets high in fiber contain plant lignan species that may be directly responsible for some observed health benefits of these diets. Enterolactone is an enterobacterial breakdown product of plant lignans that is absorbed and can be measured in human serum. Enterolactone and other ligans and phytoestrogens have been associated with a reduced risk of acute coronary events, hormone-dependent cancers, and possibly osteoporosis. Recent observations have shown an inverse association between serum enterolactone levels and serum isoprostane levels. This association implies a protective effect against oxidative injury associated with the dietary lignans themselves, enterolactone, or some intermediate in this pathway.