When new mumps vaccine strains are introduced, studies on their immunogennicity should be carried out in both industrialized and developing countries. The field effectiveness of vaccines, especially newer strains, needs to be monitored. A more difficalt task is to establish and main sufficiently sensitive monitoring systems that can provide reliable data on rare adverse events. In countries where mumps vaccine has been in use for many years, there is a needs for contineued study of the duration of protection follwing vaccination in childhood, particularly if there is little natural boosting from exposure to wild mumps virus.