The most common site of endometriosis is located in the ovaries, along with the uterosacral ligament, the recto-uterine sulcus, the lower posterior uterine wall and the pelvic peritoneum. When the lesion is located in the ovaries, single or multiple cysts of variable size can usually be seen, located in one or both ovaries. These cysts are also known as ovarian chocolate cysts because they contain old blood inside and happen to be chocolate-like and sticky. Due to the repeated bleeding inside the cysts during the menstrual cycle, they are prone to some inflammatory reactions and fibrotic adhesions with the surrounding pelvic tissues, and therefore are one of the causes of infertility in patients. When the lesion is located in the posterior vaginal fornix, various clinical manifestations such as painful intercourse and tenderness nodules are often observed. Also the disease in some cases very rarely occurs in the rectum, bladder, appendix and even the lungs are possible.