Objective. To study the eutopic and ectopic endometrium in patients with histologically confirmed external genital endometriosis. Material and methods. A comparative morphometric study of eutopic and ectopic endometrium was conducted in 30 patients of childbearing age with histologically confirmed peritoneal endometriosis that was detected during endoscopic surgery and biopsy of the lining of the uterus. Results. It has been found that the thickness of cytoplasmic membrane in eutopic endometrium was 41.4% more than in ectopic endometrium, the thickness of nuclear membrane was 8.5% larger. The area of nuclei and the size of nucleoli in eutopic endometrium were 47.2 and 6.7% larger, respectively. Conclusions. In case of peritoneal endometriosis, heterotopias, which are morphologically inhomogeneous with endometrium of the uterine cavity, form on peritoneum.