An autopsy case of anaplastic ganglioglioma in the brain stem of a 12-year-old girl is reported. The ill-defined tumor involved the right cerebellar peduncle, medulla oblongata and upper cervical spinal cord, and showed mixed proliferation of many ganglioid cells and atypical pilocytic astrocytes with necrotic areas. Immunohistochemical studies revealed the presence of chromogranin A in most ganglioid cells and of metenkephalin in some large ganglioid cells. Glial cells were positive for glial fibrillary acidic protein and vimentin. Ultrastructurally, numerous dense-core granules of 90-220 nm in diameter were demonstrated in ganglioid cells and abundant glial filaments in glial cells. High neurosecretory activity in neuronal cells, suggested by chromogranin-immunoreactivity and dense-core granules, seems to be the most characteristic property of ganglion cell tumors.