Endometriosis is an estrogen dependent disease, which causes chronic inflammation and may lead to pelvic pain and infertility. Women with endometriosis have a 1.5 to 2-fold risk for ovarian epithelial cancer. The risk is increased especially for the endometrioid and clear cell histological types of ovarian carcinoma. Endometriosis and its atypical form are often found in the proximity of these cancers, and molecular changes similar to those in cancer have been detected in nearby endometriosis. The risk of cancer is associated with ovarian endometriosis, i.e. endometriomas and is increased by prolonged disease and/or older age at diagnosis and infertility. The progression of endometriosis to cancer is usually slow, possibly enabling diagnosis at an early stage of the disease.