Mature cystic teratoma is one of the most common kinds of ovarian tumor, and immature teratoma is a rare tumor, representing less than 1% of all ovarian teratomas. Although there are some reports about familial occurrences of ovarian tumors, literature concerning the clinical cases of monozygotic twins is rare. We report the 5-year clinical courses of a 12-year-old Japanese girl with a recurrent bilateral ovarian mature teratoma and her monozygotic twin with a unilateral ovarian mature teratoma and contralateral ovarian immature teratoma. This is the first clinical report on mature and immature teratomas of the ovary in monozygotic twins. Our cases support future clarification of the molecular mechanism and pathogenesis of teratoma.