Cerebellar glioblastoma multiforme (GBM) is a rare entity in adults and an extremely rare entity in children. Approximately 30 cases have been reported in the literature. The authors report the case of a histologically confirmed cerebellar GBM presenting initially as supra- and infratentorial gliomatosis cerebri. Acute disseminated encephalomyelitis had been diagnosed in the patient and that diagnosis remained until near the end of his treatment. This case underscores the need for recognizing the clinical presentation of gliomatosis cerebri and multifocal GBM in the pediatric subpopulation thought to harbor demyelinating disease.