Benign retroperitoneal cysts (RPCs) of müllerian type may present as a pelvic mass in a perimenopausal or postmenopausal woman, and thus clinically can mimic an ovarian malignancy. We report three women, aged 73, 45, and 68 years, who presented with large RPC masses ranging from 8 to 17 cm in diameter. Histologically, the cysts were lined by benign müllerian-type epithelium: serous epithelium in two patients and mucinous epithelium in one patient. A malignant component was not present in any of the cases. Preoperative diagnosis in two patients was ovarian carcinoma based on clinical and computerized tomography findings. Two patients are alive without evidence of disease 44 and 56 months following surgical excision, whereas one patient died of perioperative complications unrelated to the cystic lesion. Similar previously reported cases and their possible pathogenetic mechanisms are discussed. Although the müllerian type of RPCs behave in a benign manner, a malignant component has to be searched for. Thus, complete excision with careful histologic examination rather than drainage or marsupialization is recommended.