Abstract Background: Gonadoblastomas are very rarely found in children, considering that primary tumors of the ovaries are uncommon such patients. Case: This report describes a case of gonadoblastoma that was found and treated in a 4½-month-old girl. She had a cystic ovarian mass of 5–6 cm in diameter, which initially appeared as a follicular cyst without signs of being complicated. An operation was performed to access the mass. After this first operation, histopathology testing revealed the presence of a gonadoblastoma in the cyst wall while the patient's karyotype was and is 46 XX (normal female type). The patient then underwent a salpingo-oophorectomy to remove the tumor. Results: There were no dysgenetic gonads in this patient. Her prognosis was good. Conclusions: Gonadoblastoma is a rare tumor in children, occurring either in intersex disorders or in children with abnormal gonads. No data are published on the race distribution of this disease. These tumors are normally more frequent in phenotypic females who present with a genetic karyotype with an Y chromosome. In particular, 80% of gonadoblastomas occur in phenotypic females with intra-abdominal testis, while in the remaining 20% of these patients the tumors occur in patients who have other conditions involving intersexuality. For these reasons, this case was very anomalous and rare and, prior to this report, has never been described in literature. (J GYNECOL SURG 28:165)