To determine the origin of benign cystic teratomas of the ovary, chromosome-banding studies were done on normal tissues and teratomas from five patients. The normal tissues were heterozygous (+/-) for 17 chromosome polymorphisms at or near the centromere, whereas the teratomas were uniformly homozygous (+/+ or - /-). These findings and those employing electrophoretic variants indicate that ovarian teratomas are parthenogenic tumors that arise from a single germ cell after the first meiotic division. (N Engl J Med 292:63–66, 1975)