The authors report a rare case of retroperitoneal leiomyosarcoma in a 49 year-old woman, with a palpable mass in the left lower abdomen. There is a large neoplasm which was soft and movable, looking a pelvic mass. Abdominal and pelvic CT scan showed a 11.0x9.2x7.5 cm-sized lobulated heterogenous enhancing mass with multifocal necrotic portion near the left ovary. Laparotomy and resection of the mass was performed. Microscopic examination revealed 15 mitoses per 10 high power fields. She was diagnosed as primary leiomyosarcoma originated from the left common iliac vein, stage I (IB). The best treatment of these neoplasms is complete surgical excision, and no therapeutic benefit has been derived from chemotherapy or radiotherapy. Though metastases are occasional, local recurrences can be taken into consideration and, after a careful tumoral re-staging, they can be resected once more. Therefore, careful follow-up is necessary on the basis of neoplastic grading, extension and involvement of the adjacent structures.