Endometriosis is a dissemination of endometrial-like tissue outside the uterine cavity, responsible for pain and impaired fertility in women of childbearing age. Although endometriosis generally occurs in the pelvis, it can be located further away. We describe the case of a 35-year-old woman who was admitted for further evaluation of a cystic mass of the liver that had invaded the right ventricle and caused pain. Serum levels of the tumor markers CA 125, CA 15-3 and CA 19-9 were elevated. The tumor was resected with a small part of the right ventricle free wall, the diaphragm and the left liver lobe. A histological analysis confirmed that the mass was a benign endometrial cyst. The postoperative course was uneventful and the patient remains asymptomatic with 5year follow-up. A diagnosis of endometriosis should be considered for thoraco-abdominal cystic masses associated with menses-related pain in women of childbearing age.