Summary Introduction: Proinflammatory and prooxidative environment in the peritoneal cavity may be involved in the pathogenesis of endometriosis. Imbalance between reactive oxygen species levels and the antioxidant capacity leads to oxidation of low-density lipoproteins (LDL). The importance of oxidized LDL (Ox-LDL) in the development of atherosclerosis is well recognized. Aim: The aim of our study was to evaluate for the presence of ox-LDL in the peritoneal fluid (PF) of women with and without endometriosis. Material and Methods: A total of 60 women who underwent laparoscopy were divided into groups: endometriosis sufferers with minimal to mild (n 20) and moderate to severe (n 20) stages, and the reference group (n 20) with functional follicle ovarian cysts. Oxidized LDL levels were determined in the PF using enzyme immunoassay. Results: Oxidized LDL levels were detectable in all peritoneal fluid samples. Significantly increased levels of ox-LDL were observed in PF of women with stage III/IV endometriosis compared to the reference group (p=0.03). However, peritoneal fluid ox-LDL concentrations did not differ significantly between patients with minimal/mild and women with moderate/severe stage of the disease (p=0.2). No significant difference in the PF ox-LDL concentrations was also found between women with stage I/II endometriosis and patients with follicle cysts (p=0.3). Conclusions: Increased peritoneal fluid ox-LDL levels observed in women with advanced-stage endometriosis suggest the important role of oxidative stress in the pathogenesis of the disease.