The ginandroblastoma is an extremely rare ovarian tumor which shows components of granulosa cells and Sertoli/Leydig cells. We describe a case of a twelve-years-old girl who presented abnormal uterine bleeding and progressively growing intraabdominal mass associated with menorrhagia, CA-125 60.4 UI/mL and extension studies reporting cystic mass in the left ovary. She underwent oophorectomy. Pathological study shows a multilocular tumor filled with serous material. Many Call-Exner bodies were observed in the histopathological analysis, 45% of Sertoli/Leydig cells. Immunohistochemistry was reactive for inhibin, calretinin and pCK while AE1/AE3 and CD99 markers were negative. This is the first case report about a multiloculated gynandroblastoma, negative for CD99 in a 12-years-old girl. Thus, the study of this clinical case represents a systematic approach for tumors of the sex cord cells.