A series of 35 cerebellar medulloblastomas was studied using light microscopy and immunohistochemistry for localization of glial fibrillary acidic protein, an antigen specific for neuroglia. Most medulloblastomas occurred in children less than 15 years of age (77.1%), especially in the first decade of life (65.7%), with the peak incidence (40.0%) between 6 and 10 years. The rest were found in adults beyond 15 years of age (22.9%). The male to female ratio was 4:3. Ependymal differentiation was observed in 100% of medulloblastomas, astrocytic differentiation in 88.6%, oligodendroglial differentiation in 34.3%, glioblastomatous differentiation in 8.9%, and neuronal differentiation in 8.9%. These data suggest that medulloblastoma is a primitive (stem cell) neuroepithelial neoplasm with the capacity of differentiating along both neuroglial and neuronal directions. Excessive mucin production was encountered in one medulloblastoma. Leptomeningeal invasion occurred in 34.3% of medulloblastomas and endothelial hyperplasia in 28.6%. One medulloblastoma (2.9%) spread postoperatively to several bones.