Cytogenetic analysis of a malignant teratoma of the testis revealed a hypotriploid karyotype with the presence of an isochromosome of the short arm of chromosome 12, i(12p), as the only structural abnormality. Most cells contained 3 copies of chromosome 12 and 2 copies of the i(12p) isochromosome. Thus, the genetic material located on the short arm of chromosome 12 was multiplied in the teratoma cells, being represented by 7 copies compared to 2 to 3 copies of other chromosomes. This finding confirms the reports that the i(12p) isochromosome is a specific chromosomal abnormality in cases of malignant testicular tumors.