Fertility preservation for a patient with advanced immature teratoma of the ovary is reported. The patient, a 29-year-old woman, delivered a healthy baby after having had ovarian immature teratoma, grade 3, uncertain stage, at 13 years of age. She was initially treated with unilateral salpingo-oophorectomy and a contralateral wedge resection for tumor invasion, followed by a 6-course cisplatin+vinblastine+bleomycin regimen, a second operation, and an additional 6-course etoposide and cisplatin regimen with complete remission. The patient delivered a healthy baby 16 years after the initial treatment. Based on this successful case, intensive fertility-preserving surgery followed by chemotherapy, even in advanced-stage immature teratomas of the ovary, may be effective in preserving the reproductive function of women with malignant immature teratomas of the ovary.