Intramedullary gangliogliomas are indolent tumors of the spine with rare cases of malignant transformation. Conus medullaris gangliogliomas are extremely rare but are usually associated with good clinical outcome after gross total resection. We describe the first reported case of a ganglioglioma of the conus medullaris that progressed to glioblastoma multiforme 1 year after subtotal resection and radiotherapy. The paraplegic patient lacked neurological function below the level of the lesion and thus was treated with cordectomy caudal to the root entry zones of the T10 sensory roots.