An unusual massive benign cystic monodermal teratoma of the ovary in a 14-year-old girl is reported. The multilocular cyst measured 80 x 70 x 80 cm and contained 23 l of fluid. The cyst wall composed of purely mature central nervous system tissue with a prominent microvasculature resting on a thin rim of ovarian cortical tissue. Immunohistochemical stains supported the diagnosis. To the best of our knowledge only one such case has been described in the literature.