A gonadal tumor was diagnosed in the first months of life in a patient with genital ambiguity, a 45,X/46,XY karyotype, and mixed gonadal dysgenesis. Gonadal biopsies at the age of 3 months revealed dysgenetic testes and a gonadoblastoma on the right testis. Even though gonadal tumors are rare in childhood, this case indicates that prophylactic removal of dysgenetic gonads should be performed as early as possible, especially when the female sex is assigned to a patient with a Y-chromosome sequence.