Endometriosis is a disorder associated with a general inflammatory response in the peritoneal cavity. Oxidative stress is a potential factor involved in the pathophysiology of this disease, and reactive oxygen species (ROS) are implicated in this process. Indeed, in healthy individuals, ROS and antioxidants are in balance, but when balance is tipped toward an overabundance of ROS, oxidative stress occurs and can impact the entire reproductive lifespan of a woman. Reactive oxygen species are intermediaries produced by normal oxygen metabolism but are known to have deleterious effects. Excessive release of ROS induces cellular damage and alters cellular function by regulating protein activity and gene expression, leading to harmful effects. To protect themselves, cells have developed antioxidant systems to limit production of ROS, inactivate them, and repair cell damage. Understanding of the control of hemoglobin, heme, and iron-induced redox balance in endometriosis led us to propose a number of hypotheses to explain why oxidative stress is induced in case of pelvic endometriosis. Erythrocytes, apoptotic endometrial tissue, and cell debris transplanted into the peritoneal cavity by menstrual reflux and macrophages have all been cited as potential inducers of oxidative stress. Erythrocytes are likely to release pro-oxidant and proinflammatory factors, such as hemoglobin and its highly toxic by-products heme and iron, into the peritoneal environment. Iron and heme are essential to living cells, but unless appropriately chelated, free iron, and to a lesser extent heme, play a key role in the formation of deleterious ROS.