Epithelial ovarian tumors are uncommon before 20 years of age and rarely occur before puberty. The vast majority of these tumors are benign, and few cases of malignant and borderline tumors are described. We report a case of a 4-year-old premenarchal girl, previously healthy, who presented with two abdominal masses. Laboratory analysis of blood showed elevation of the serum level of the CA-125. Ultrasonographic examination disclosed bilateral cystic ovarian masses. Laparoscopic exploration revealed bilateral ovarian multicystic masses with retro-uterine peritoneal implant. Bilateral salpingo-oophorectomy with implant resection was performed. Histologic findings were consistent with a serous borderline tumors of both ovaries and the peritoneal implant was of the non-invasive type. There is no evidence of recurrence at 3-year follow-up. To our knowledge, there are only four cases of ovarian borderline tumors in premenarchal girls reported in the English literature: three of the mucinous type and only one of the serous type.