Gangliosides, which are sialylated glycosphingolipids, are the major class of glycoconjugates on neurons and carry the majority of the sialic acid within the central nervous system (CNS). To determine the role of ganglioside synthesis within the CNS, mice carrying null mutations in two critical ganglioside-specific glycosyltransferase genes, Siat9 (encoding GM3 synthase) and Galgt1 (encoding GM2 synthase), were generated. These double-null mice were unable to synthesize gangliosides of the ganglio-series of glycosphingolipids, which are the major ganglioside class in the CNS. Soon after weaning, viable mice developed a severe neurodegenerative disease that resulted in death. Histopathological examination revealed striking vacuolar pathology in the white matter regions of the CNS with axonal degeneration and perturbed axon–glia interactions. These results indicate that ganglioside synthesis is essential for the development of a stable CNS, possibly by means of the promotion of interactions between axon and glia.