A 27-year-old woman visited our hospital with chief complaints of abducens nerve palsy and cerebellar symptoms. On computerized tomographic scanning and magnetic resonance imaging, a tumor with strong enhancement was found on the dorsal side of the medulla oblongata. A tumor was excised by suboccipital craniotomy and C1 laminectomy. Histologically, many Rosenthal fibers together with pilocytic tumor cells were found in some regions, but a very high Ki-67 labeling rate accompanied by cells with nuclei of irregular size and giant cells was observed in other regions. The tumor was diagnosed as malignant pilocytic astrocytoma originating from pilocytic astrocytoma by transformation. The biological behavior of pilocytic astrocytoma is obscure in several respects. We report our experience of a case of malignant pilocytic astrocytoma that developed in the brain stem and progressed extremely rapidly.