Dysgerminoma is the most common malignant ovarian germ cell tumor and shares histological and immunophenotypical features with its testicular counterpart, seminoma. Chromosome 12p abnormalities are genetic hallmarks of testicular seminomas. Little is known about these genetic changes in dysgerminoma. We performed dual color fluorescence in situ hybridization (FISH) analyses with a centromeric alpha-satellite probe for chromosome 12 and a subtelomeric probe for 12p on paraffin-embedded tissue sections from 21 dysgerminomas and two gonadoblastomas. Chromosome 12p abnormalities were detected in 81% of dysgerminomas. In all, 57% of cases had only isochromosome 12p and 5% had only 12p overrepresentation. In all, 19% had both isochrome 12p and 12p overrepresentation. Gonadoblastomas were negative for isochromosome 12p or 12p overrepresentation. Chromosome 12p abnormalities are common in dysgerminoma of the ovary. FISH analyses for chromosome 12p abnormalities may be a useful diagnostic adjunct for confirming the diagnosis of dysgerminoma and for distinguishing it from nongerm cell malignancies that enter into the differential diagnosis.