Endometrioid ovarian cyst is a pathological growth of tissue, morphologically identical to the inner layer of the uterus (endometrium). This is one of the clinical forms of genital endometriosis. This disease mostly affects women of fertile age. Only 6% of patients, who are diagnosed with an endometrioid cyst for the first time, are over 50. In 70-90% of cases endometrioid ovarian cyst is accompanied by endometriosis of another localization, or another disease of the reproductive system.