BACKGROUND Endometrial stromal sarcoma (ESS) is an uncommon malign neoplasm, and its occurrence outside the uterus is extremely rare in the absence of metastasis or extension of a primary uterine neoplasm. When arising in the pelvis or abdominal cavity, ESS is associated with uterine adnexa or serosal surface of various organs. METHODS We present the case of a 46-year-old woman with lower abdominal pain and regular menstruation who underwent laparotomy after a diagnosis of pelvic mass mimicking a right adnexial tumor. Exploration of the pelvis revealed a retroperitoneal mass of 15 cm in diameter in the right illiac fossa without accompanying pelvic or paraaortic lymphadenopathy. Uterus and ovaries were bilaterally normal in size. The pathology showed low-grade ESS of the uterus with direct spread to retroperitoneum without serosal metastases. CONCLUSIONS This case shows that despite its well-known good prognostic nature, low-grade ESS may behave as an aggressive malignancy.