A three-year-old boy with a progressive history of headache, vomiting and ataxia in the course of 2 months, was admitted on August 1983, when he was lethargic. Neurological examination revealed dysphagia, scanning speech and tremor in the bilateral hand. CT scan showed a very large enhanced mass in the center of posterior fossa with central necrosis in it and the dilatation of whole ventricular system. Suboccipital craniectomy was immediately performed and the tumor that occupied the vermis and invaded into both cerebellar hemisphere was subtotally removed. Postoperative irradiation was well performed: 4140 rads to the whole brain and 3162 rads to the spinal cord. However, 5 months later, facial palsy in the left side and progressive ataxia became prominent. CT scan showed multiple enhanced masses in the left trigonum and right anterior horn of the lateral ventricles and in the left cerebellopontine angle. In spite of chemotherapy, the patient had a down-hill course, especially after the ventricular hemorrhage, and died on June 9th, 1984. Histologically, the tumor had a lobulated appearance with an aggregation of tumor cells encircled by vascular septae. The cells within lobules generally had vesicular nuclei, which were arranged in parallel row. Occasionally smaller hyperchromatic cells with scant cytoplasm were present along the vascular septae. Reticulin was present within the septa, but was not observed within the lobules. Scattered astrocytic cells and processes were identified within the lobules by the immunoperoxidase technique for GFAP. The fibrillary cytoplasmic processes within the lobules were stained by immunoperoxidase technique for neurofilament (68K).(ABSTRACT TRUNCATED AT 250 WORDS)