Ovarian tumors constitute the most common gynecological neoplasms during childhood. Unlike in adult women, ovarian epithelial tumors are uncommon in young girls and extremely rare prior to menarche. To our knowledge this is the first report of a borderline malignant mucinous tumor of the ovary in a premenarchal girl. It was successfully treated by conservative surgery. In our review of ovarian neoplasms in girls under 20 years of age, epithelial tumors were found in 19.3%, with a malignancy rate of 15.9%. Of these malignant tumors, 39.4% were of the mucinous type and 30.3% were borderline malignant. The occurrence of such a tumor at this age underscores the possibility of malignant transformation of the ovarian epithelium even prior to menarche. Its mucinous nature confirms the predominance of the mucinous subgroup in young girls. Hypotheses on the origin of mucinous tumors are discussed.