A pleocytosis of the CSF occurred in all of 10 children with mumps meningitis and persisted for weeks and months in some patients. Oligoclonal IgG proteins were detected in the cerebrospinal fluid (CSF) during the 2nd week after onset of meningitis or later in 4 out of 10 patients, and could be detected as late as 11 and 12 months after meningitis in 2 patients. Evidence is presented that the oligoclonal IgG represents mumps virus-specific antibody synthesized locally in the brain. Oligoclonal virus antibodies were demonstrated also in serum samples. The persistence for weeks and months of pleocytosis and oligoclonal IgG virus antibody in the CSF may imply that a virus infection in some cases persists in the brain in spite of apparently complete clinical recovery.