Inflammatory processes have been hypothesized to mediate some of the clinical sequelae associated with endometriosis. The peritoneal fluid (PF) of women with endometriosis is known to contain more inflammatory cells and their associated cytokines, chemokines, and growth factors. This work provides strong evidence for oxidative stress in the PF of women with endometriosis. 1) The low density lipoprotein (LDL) isolated from the PF of subjects with endometriosis shows a small but detectable increase in electrophoretic mobility compatible with mildly oxidized LDL compared with LDL isolated from the plasma of the same subjects and PF of controls. 2) Isolated PF-LDL of endometriosis subjects is more readily oxidized in vitro than PF-LDL of controls, or LDL isolated from plasma. 3) Vitamin E content is significantly lower in endometriosis PF compared with controls, and compared with plasma of women with endometriosis and controls. No difference is seen between plasma and PF of control subjects. 4) The ratio of phosphatidylcholine/lyso phosphatidylcholine (Ptd/lyso PtdCho) in the PF of endometriosis subjects is significantly lower compared with PF of controls. Taken together, these data provide strong evidence for a pro-oxidant environment in the peritoneal cavity of women with endometriosis. Lyso PtdCho, a product derived from phospholipase A2 action on peroxidized phosphatidylcholine and a potent chemotactic factor for monocytes and T-lymphocytes, is elevated in endometriosis. We hypothesize that the increased presence of lipid peroxidation products in the PF of endometriosis subjects may, at least partly, account for the recruitment of leukocytes, the increase in macrophage activation, the secretion of monocyte--macrophage-derived cytokines, and the endometrial growth-promoting activity associated with endometriosis.