Measles can persist in the central nervous system and cause subacute sclerosing panencephalitis (SSPE), a progressive disease that is almost always fatal. The clinical findings of SSPE include behavioral changes, ataxia, seizures, and mental-motor deterioration that begins several years after natural infection with the virus. Measles antibody is found in the spinal fluid, and its presence is particularly useful in establishing the diagnosis. The mechanisms of pathogenesis of SSPE apparently involves the selection of clones of virus, which do not replicate to become complete virus particles. These "suppressed" viruses are able to multiply and spread from cell to cell. The immune system of the patient is unable to clear this infection. Several hypotheses are advanced to explain these events. Further studies are needed, however, to develop a complete understanding of the pathogenesis of these diseases. Fortunately, SSPE is disappearing in the United States. This apparently is related to the widespread use of measles vaccines and to the resulting low frequency of natural measles infections.