Subacute sclerosing panencephalitis (SSPE), a neurodegenerative disease caused by a persistent "slow virus infection" with a mutated measles virus, is endemic in much of the developing world. Its incidence will increase in the USA, not only in immigrants, but also because of the 1988-1990 measles epidemic. This report reviews the pathogenesis, clinical and laboratory diagnosis, and future perspectives in treatment and prevention.