A 16-year-old woman presented with a non-germinomatous germ cell tumor in the neurohypophysis manifesting as progressive visual disturbance, amenorrhea, hydrodipsia, and polyuria. Her serum α-fetoprotein and human chorionic gonadotropin levels were elevated. She experienced sudden, rapid visual deterioration and underwent emergency partial tumor removal to decompress the optic nerves. Her vision subsequently improved. Histological examination of the surgical specimens confirmed immature teratoma. She received chemotherapy (ifosphamide 900 mg/m², cisplatin 20 mg/m², etoposide 60 mg/m²) for 5 consecutive days. Although the tumor marker levels decreased remarkably, her vision again declined rapidly due to enlargement of the tumor after the first course of chemotherapy. A second radical operation resulted in vision improvement. The tumor specimen showed only mature teratoma elements. This phenomenon, called the growing teratoma syndrome, is very rare in intracranial non-germinomatous germ cell tumors.