Objective - to search for specific markers for the diagnosis of external genital endometriosis through proteomic analysis. Material and methods. The investigation enrolled 88 reproductive-aged patients. Group 1 consisted of 28 patients with Stages I-II external genital endometriosis; Group 2 comprised 30 patients with Stages III-IV of the disease; Group 3 (a control group) included 30 patients without endometriosis. The proteomic analysis was carried out using high-resolution two-dimensional electrophoresis and time-of-flight mass spectrometry. The peritoneal fluid and serum levels of decoration proteins were estimated by an enzyme immunoassay. Results. Changes were found in the proteomics spectrum of peritoneal fluid in endometriosis. In the latter, there was a significant increase in the level of apolipoprotein A-IV, sex hormone-binding globulin, and complement component C2 in the peritoneal fluid and serum. Whether the found changes may be implicated in the pathogenesis of endometriosis was discussed. Conclusion. The above proteins may be proposed as informative diagnostic markers for this disease. The authors declare no conflicts of interest.