Are protein oxidative stress markers [thiols, advanced oxidation protein products (AOPP), protein carbonyls and nitrates/nitrites] in perioperative peritoneal fluid higher in women with histologically proven endometriosis when compared with endometriosis-free controls? Protein oxidative stress markers are significantly increased in peritoneal fluids from women with deep infiltrating endometriosis with intestinal involvement when compared with endometriosis-free controls. Endometriosis is a common gynaecologic condition characterized by an important inflammatory process. Various source of evidence support the role of oxidative stress in the development of endometriosis. We conducted a prospective laboratory study in a tertiary-care university hospital between January 2011 and December 2012, and included 235 non-pregnant women, younger than 42 year old, undergoing surgery for a benign gynaecological condition. After complete surgical exploration of the abdomino-pelvic cavity, 150 women with histologically proven endometriosis and 85 endometriosis-free controls women were enrolled. Women with endometriosis were staged according to a surgical classification in three different phenotypes of endometriosis: superficial peritoneal endometriosis (SUP), ovarian endometrioma (OMA) and deeply infiltrating endometriosis (DIE). Perioperative peritoneal fluids samples were obtained from all study participants. Thiols, AOPP, protein carbonyls and nitrates/nitrites were assayed in all peritoneal samples. Concentrations of peritoneal AOPP were significantly higher in endometriosis patients than in the control group (median, 128.9 µmol/l; range, 0.3-1180.1 versus median, 77.8 µmol/l; range, 0.8-616.1; P < 0.001). In a similar manner concentrations of peritoneal nitrates/nitrites were higher in endometriosis patients than in the control group (median, 24.8 µmol/l; range, 1.6-681.6 versus median, 18.5 µmol/l; range, 1.6-184.5; P < 0.05). According to the surgical classification, peritoneal fluids protein AOPP and nitrates/nitrites were significantly increased only in DIE samples when compared with controls (P < 0.001 and P < 0.05; respectively), whereas the others forms of endometriosis (SUP and OMA) showed non-statistically significant increases. We found positive correlations between peritoneal fluids AOPP concentrations, nitrites/nitrates levels and the total number of intestinal DIE lesions (r = 0.464; P < 0.001 and r = 0.366; P = 0.007; respectively). Inclusion of only surgical patients may constitute a possible selection bias. In fact, our control group involved women who underwent surgery for benign gynaecological conditions. This specificity of our control group may lead to biases stemming from the fact that some of these conditions, such as fibroids, ovarian cysts or tubal infertility, might be associated with altered peritoneal proteins oxidative stress markers. We demonstrate the existence of a significantly increased protein oxidative stress status in peritoneal fluid from women with endometriosis especially in cases of DIE with intestinal involvement. This study opens the way to future more mechanistics studies to determine the exact role of protein oxidative stress in the pathogenesis of endometriosis. Even if an association does not establish proof of cause and effect, these intrinsic biochemical characteristics of endometriosis may lead to the evaluation of therapeutic approaches targeting oxidative imbalance. No funding was used for this study. The authors have no conflict of interest.