BACKGROUND: Pelvic retroperitoneal leiomyomas arising from the rectal wall are rare. We present a case of a giant retroperitoneal leiomyoma mimicking bilateral solid adnexal masses in a postmenopausal woman. CASE: A 54-year-old postmenopausal woman presented with a large abdominopelvic mass. At surgery, the uterus was displaced anteriorly by a large retroperitoneal mass. The rectosigmoid colon was noted to course through the retroperitoneal mass. The patient underwent complete excision of the retroperitoneal mass along with a rectosigmoid resection of the involved colon with primary reanastomosis. Histopathology showed a leiomyoma arising from the muscularis propria of the rectum wall. CONCLUSION: Retroperitoneal masses that extend into the pelvis may mimic adnexal masses and, therefore, represent a rare finding at gynecologic surgery.