Central nervous system infection caused by mumps virus is reviewed. Aseptic meningitis, acute encephalitis, chronic encephalitis, hydrocephalus, ataxia, transverse myelitis, Guillain-Barré syndrome and deafness are noted as central nervous system complications of mumps. Among these complications, acute and chronic encephalitis, hydrocephalus, transverse myelitis and deafness occasionally result in severe sequelae. Aseptic meningitis due to mumps vaccine strain increased significantly after the introduction of trivalent measles, mumps and rubella vaccination in Japan. Recently, a differentiation method of mumps vaccine strain from wild viruses has been established using molecular analysis. Mumps virus infection is still one of the most common communicable viral infections, although it is preventable by vaccination. We emphasize the importance of mumps vaccination to prevent such severe complications as encephalitis and deafness.