Ovarian teratoma, comprising cystic mature teratomas and malignant immature teratomas, is a common cancer of the female reproductive tract that tends to occur in young women, forms a relatively large tumor mass, and displays differentiated or primitive tissue from 2 or 3 germ cell layers. Ovarian teratomas contain highly heterogeneous cell morphology, may disseminate to distant tissues, and occur bilaterally in 10-15% of cases. By exome sequencing, we have found a unique tumor etiology that is defined by the absence of significant point mutations, but widespread copy-neutral loss of heterozygosity affecting thousands of genes in each tumor genome. We also utilize loss of heterozygosity patterns to infer the clonal relationship between morphologically contrasting regions of immature teratomas, and uncover strong evidence that failure of meiosis I, meiosis II, or whole genome duplication is the defining genetic event of ovarian teratoma.