Since the introduction of the mumps vaccine in the U.S.A., the number of cases and those of associated complications of mumps have decreased, but the rates of complications have remained unchanged. The average age of those falling ill to mumps has increased because of the high degree of immunity among younger, immunized children. Rates of severe complications (encephalitis, death) are higher when the illness takes place at any age beyond the first decade of life, but increase particularly sharply in adulthood. Though an upward shift in age of those getting mumps has not yet been recorded in Switzerland, the relatively high rate of immunization among young children and a large group of susceptible older individuals means that similar conditions to those in the U.S. are present. We should continue to immunize susceptibles with the most effective available mumps vaccine because of this anticipated shift and the higher rates complications in older patients. The cost-benefit ratio of immunization compared to illness also speaks favorably for immunization.