Background: Mixed gonadal dysgenesis is a rare congenital and challenging condition, characterized mainly by 45,X/46,XY karyotype mosaicism, asymmetrical gonadal development and various internal and external genital anatomy. Because of frequent disorder of genital development and a higher risk of germ cell neoplasia, management of these patients is complex and requires multidisciplinary approach.Case: We present a 45,X/46,XY mixed gonadal dysgenesis patient diagnosed with gonadoblastoma in both gonads after bilateral gonadectomy at 1 year of age.Conclusions: Because of high risk for malignant transformation, gonadectomy of a streak-like gonad and biopsy with orchidopexy or gonadectomy of a dysgenetic testicle is recommended at an early age.