A mass in the left annexal zone was discovered in a 56-year-old woman at the Department of General Surgery and was diagnosed as ovarian cancer. After the operation the mass appeared histologically to be retroperitoneal leyiomiosarcoma and because of residual disease, confirmed by computed tomography (CT) and nuclear magnetic resonance (NMR), complementary radiotherapy was carried out. Restaging supported the persistence of the tumor and so a second laparotomy was performed with complete tumor resection; the pathologic diagnosis was retroperitoneal benign schwannoma. The importance of careful preoperative imaging, such as echography, CT, NMR, arteriography and urography should be stressed for a correct clinical and surgical approach. Moreover, considering that in some selected clinical cases these tumors could be confused with others deriving from contiguous organs and structures, a different surgical approach may be needed together with dedicated and expert surgeons.