Phenotypically female patients with a (mosaic) XY karyotype are at high risk to develop gonadoblastoma with potential progression to dysgerminoma. We studied a Turner's syndrome patient with a composite ovarian neoplasm of a gonadoblastoma, a dysgerminoma, and a mucinous cystadenoma. Nonradioactive in situ hybridization showed that the patient had a XO/XY genotype with deletion of part of Yq. Molecular analysis located the chromosomal breakpoint in deletion interval 6, indicating that potential genes responsible for the development of gonadoblastoma may be located on the short arm of the Y chromosome or on the long arm, centromeric of deletion interval 6. Moreover, using the XO/XY mosaicism as a clonal marker, the dysgerminoma and the mucinous cystadenoma were shown to be of independent origin. Therefore, in this case, we find support for the hypothesis that mucinous cysts with gastrointestinal epithelium can be of ovarian surface epithelial cell origin. This case also demonstrated that the occurrence of a composite tumor does not unequivocally imply that both components are of the same origin. Clonal analysis is required to determine the relation of the tumor constituents.