In this article we present a case of retroperitoneal schwannoma localized in the pelvic cavity with complete cystic degeneration, mimicking, ovarian carcinoma. Case: A 25-year-old nulligravid woman was admitted to our gynecology out-patient clinic with the complaint of abdominal distension. right flank pain and pollakiuria. Sonographic study showed a cytic mass containing necrotic areas and filling the whole pelvic cavity. Computed tomography and colonography suggested the possible diagnosis of a right ovarian malign tumor. Laparotomy revealed a retroperitoneal cystic lesion. Microscopic examination of the pelvic mass confirmed the diagnosis of cystic degenerative schwannoma. After six months of initial surgery she is still alive without any evidence of disease. Conclusion: In the present case, it is emphasized that it is easy to make an error and misdiagnose a pelvic mass as an ovarian tumor when it is in fact a tumor from another origin. Additonally, clinicians should keep in mind that retroperitoneal schwannoma may mimic cystic ovarian carcinoma even in young women.