External genital endometriosis is characterized by high prevalence. Despite unquestionable role of oxidative stress in the development of endometriosis, its features associated with the disease progression are poorly understood. The study aimed to assess the role of oxidative protein modification (OPM) products in the disease progression. A total of 28 women with the established and histologically verified diagnosis of endometriosis took part in the study. The subjects were stratified into group 1 (stage II–III, n = 10) and group 2 (stage IV, n = 18) based on the disease stage in accordance with the American Association of Gynecologic Laparoscopists (AAGL) classification. The content of OPM products in the ectopic endometrium homogenate was determined by spectrophotometry. The following indicators were significantly higher in patients of group 2, than in patients of group 1: the total OPM product content (p = 0.040), basic aldehyde-dinitrophenylhydrazones (p = 0.039), and the total amount of ketone-dinitrophenylhydrazones (p = 0.024). Furthermore, women of group 2 showed significantly lower reserve-adaptative potential values, than patients of group 1 (p = 0.045). We found a positive correlation between the disease severity and the content of OPM products showing intergroup differences (R between 0.397 and 0.443, p < 0.05), along with the negative correlation with the reserve-adaptative potential (R = –0.388, p = 0.042) based on the correlation analysis results. According to the data obtained, the oxidative stress activity in endometriosis foci was correlated with the disease severity. Furthermore, accumulation of OPM products and depletion of the tissue adaptive capacity are typical.