In order to update our information concerning immunization coverage in Switzerland we analysed data from the public school immunization program of the canton of Aargau between 1988-90. We compared these results with those of two districts from the school year 1993-94. 97% of all the matriculating or graduating school children could be reached. As expected, the immunization rates of 98% for poliomyelitis and 96% for diphtheria/tetanus were high. Following initiation of the nationwide MMR (Measles/Mumps/Rubella) Immunization Program in 1987, coverage of preschool children with the MMR-vaccination could be improved from 44% in the late 1980's to 70-80% in the early 1990's. As a result of offering the vaccine free of charge to all matriculating and graduating children, the MMR-immunization rate could be increased to 90-94%, and the measles immunization rate even further to 97% indicating the importance of the school immunization program. This high level of acceptance in Aargau shows that an increase in the immunization rate even in infant and preschool children can be realised, if the pediatricians and the general practitioners improve their efforts. In regions with similar epidemiological conditions MMR-reimmunization in school children is to be recommended.