Background. Serous borderline tumor is a non-invasive epithelial ovarian tumor that occur at the reproductive age, present in early stage, frequently associated with infertility but it is easily curable. Although it may have symptomatic long-term recurrences, it has an excellent prognosis in spite of peritoneal spread. Among the epithelial tumors of the ovary, borderline serous tumor fall in the spectrum lying between cystadenomas (benign) and cystadenocarcinomas (malignant). Its oncological behavior is more aggressive than benign ovarian tumors but relatively less than that of malignant ovarian tumors. Since the affected age group is usually young females, preservation of fertility is an important aspect of treatment protocol that is why an accurate diagnose is an essential step in these cases. Case report. A 12-year-old girl who presented painless abdominal distension over five months was referred to institute of Mother and Child for diagnosis and treatment. She had no medical history with the exception of abdominal distension and amenorrhea. The last menstrual cycle was 3 months before the admission. Her menstrual cycle has been irregular since she experienced the menarche at the age of 12. There was no reported use of oral contraceptives, and she was not known to be sexually active. Her physical examination showed abdominal distension and a firm mass without tenderness, extending from the pelvis to the umbilicus. An USG examination revealed left sited ovarian mass. Her tumor marker analysis, CA 19-9 (2,241 U/mL) and CA 125 (274 U/mL) were highly elevated. Routine blood analyses showed normal renal and liver function with the exception of elevated alkaline phosphatase (172 IU/L). Laparotomy was performed with a midline incision and a left salpingo-oophorectomy was performed. The surgical specimen was sent to pathology laboratory. There was confirmed serous borderline tumor. Histological description: serous cystadenofibroma with focal borderline of non-micropapillary type architecture. The CA 125 and CA 19-9 levels were decreased at 3rd, 6th and 12th months of follow-up. Conclusions. In the adolescence, an early diagnosis for ovarian tumors is required for the determination of the direction of treatment. It is important to detect the possibility of malignancy in the early stage due to the effect on the future fertility and ovarian function. The goals of treatment for children and adolescents are to exterminate the disease, and restore the uterus and ovarian function for conservation of reproductive potential.