A 13-year-old girl underwent a right salpingo-o-ophorectomy and partial omentectomy for an ovarian tumor that on microscopic examination was a grade 1 immature teratoma. Mature glial implants were found in the omentum (Stage III). No additional treatment was given. Ten months later, grade 0 and grade 1 peritoneal glial implants were found on laparotomy. Chemotherapy with four cycles of vincristine/cisplatin/etoposide/bleomycin was administered. A second laparotomy 4 months later showed persistence of grade 0 and grade 1 peritoneal glial implants. The patient was well for the next 7 years, after which time she presented with a pelvic mass that on microscopic examination was a malignant neuroectodermal tumor resembling a glioblastoma multiforme. The tumor did not respond to debulking and chemotherapy, and the patient died 6 months later, 8 years after her initial presentation. This case represents the second report of malignant transformation of peritoneal glial implants.