Cytogenetic investigation of a malignant ovarian tumor diagnosed as a mixed germ-cell tumor, composed of extensive choriocarcinoma and foci of yolk-sac tumor, revealed a highly abnormal chromosomal pattern. We found a chromosome number in the hypertriploid/hypotetraploid range, and several clonal structural abnormalities, including 2 copies of an isochromosome 12p. We showed that the chromosomal pattern of this ovarian tumor is very similar to that of testicular germ-cell tumors. This finding, together with reported cytogenetic data of malignant ovarian germ-cell tumors, supports the hypothesis that ovarian and testicular germ-cell tumors are strongly related entities that may have a similar origin and pathogenetic pathway.