Mumps meningitis (MM) is the most frequent inflammatory disease of the central nervous system in children. It occurs twice as often in boys as in girls. Usually it is mild; encephalitic signs are found only in 2-3% of the cases. Even here persistent neurological signs are rare, but psychological alterations are found in about 20% of the cases. 40% of all MM cases occur in children of school age necessitating absence from school for four weeks at last. The frequency of MM makes it a financial problem too. In Viennese hospitals 4.2 million Austrian Schillings were spent yearly for hospitalization of children with MM, on average, during the years 1971-1975. The vaccination of all Viennese children born in a single year (18,000) against mumps would require an outlay of only 2.8 million Austrian Schillings. Wide-spread routine vaccination against mumps is indicated, therefore, not only for medical, but for social and economic reasons as well.