A case of a unilateral gonadoblastoma occurring in a chromatin-negative phenotypic female with 45, XO/46, XY fragment mosaic chromosome constitution is presented. The gonadoblastoma presumably arose in a testis-determined rudimentary gonad and the contralateral streak was undifferentiated, giving a typical picture of XY gonadal dysgenesis with neoplastic transformation. The condition may be gene determined and maternally transmitted either by an X-linked recessive or autosomal dominant gene with expression only in chromosomal males. Current hypotheses concerning the development of dysgenetic gonads and their potential malignant transformation are reviewed and discussed.