Endometriosis is an uncommon disease in postmenopausal women (PMW), ranging from 2% to 5% of cases, and it is very important to exclude neoplastic transformation of the endometrium. The authors would like to introduce the case of a 63-year-old overweight patient with abdominal pain associated to pain and swelling of the left inferior limb occurring for approximately six weeks. The CT X-ray of the abdomen revealed the presence of a retroperitoneal mass causing deep vein thrombosis because of extrinsic compression of the left iliac vein. Following removal of the pelvic masses with laparotomy, the histological exam revealed an endometriosis. The CT X-ray carried out after a month postoperatively revealed the root canal treatment of the left femoral vein with a considerable decrease of the thrombosis of homolateral external iliac veins. Despite the endometriosis, it is uncommon in women who have reached menopause and must be considered in the differential diagnosis of pelvic masses.