Genetics and biology of human ovarian teratomas. I. Cytogenetic analysis and mechanism of origin. One hundred and two benign, mature ovarian teratomas and two immature, malignant teratomas were karyotyped and scored for centromeric heteromorphisms as part of an ongoing project to determine the chromosomal karyotype and the genetic origin of ovarian teratomas and to assess their utility for gene-centromere mapping. Karyotypic analysis of the benign cases revealed 95 46,XX teratomas and 7 chromosomally abnormal teratomas (47,XXX, 47,XX,+8 [two cases], 47,XX,+15, 48,XX,+7,+12 91,XXXX,-13 [mosaic], 47,XX,-15,+21,+mar). Our study reports on the first cases of tetraploidy and structural rearrangement in benign ovarian teratomas. The two immature cases had modal chromosome numbers of 78 and 49. Centromeric heteromorphisms that were heterozygous in the host were homozygous in 65.2% (n = 58) of the benign teratomas and heterozygous in the remaining 34.8% (n = 31). Chromosome 13 heteromorphisms were the most informative, with 72.7% heterozygosity in hosts. The cytogenetic data indicate that 65% of teratomas are derived from a single germ cell after meiosis I and failure of meiosis II (type II) or endoreduplication of a mature ovum (type III); 35% arise by failure of meiosis I (type I) or mitotic division of premeiotic germ cells (type IV). 