Immature teratomas of the ovary represent less than 1% of all ovarian teratomas. They contain several tissues that derive from the three embryologic layers: ectoderm, mesoderm, and endoderm. We report the case of a 17-year-old girl who presented with an immature teratoma of the left ovary that recurred 3 months later as an inoperable malignant neuroepithelial peritoneal tumor resembling a glioblastoma. She died 3 weeks after the second explorative operation. The very aggressive course and fast lethal outcome could be explained by the patient's refusal to receive chemotherapy after the initial operation.