We determined the DNA ploidy and the centromeric copy number of chromosomes 7, 12, 18, and X in four cases of ovarian dysgerminoma using DNA flow cytometry and fluorescence in situ hybridization (FISH) with chromosome-specific alpha-satellite probes. The analyses were performed on nuclei isolated from paraffin-embedded tissue. The DNA index of the tumors ranged from 1.75 to 2.08 (near tetraploid). The FISH analysis demonstrated five copies of chromosome 7 and four copies of chromosome 12 in most tumors. The copy number of chromosome 18 ranged from two to four. The X chromosome was present in three copies in most tumors. These data show that the aneuploidy profile of dysgerminoma is similar to that of testicular seminoma. Overrepresentation of chromosomes 7 and 12 and underrepresentation of chromosome 18 are characteristic cytogenetic features of seminoma. Seminoma and dysgerminoma share the same chromosomal marker, an isochromosome i(12p). Our data suggest that these tumors are also characterized by a similar, nonrandom pattern of chromosome gains and losses.