In 1984, the Health Council of the Netherlands advised that a new vaccination strategy against measles, mumps and rubella (MMR) should be initiated. The use of a combined MMR vaccine, to be given at 14 months and 9 years of age was recommended. An analysis of this strategy based on mathematical models, predicted that rubella and mumps and very probably measles will be eliminated in The Netherlands before 1990. The use of a combined MMR vaccine has a number of logistical and financial advantages over immunization with separate vaccines. Another factor to be considered is of course the mutual interference regarding immune response or immunization reactions. Less obviously, and often not recognised as an important factor, is the possibility of an adverse effect on the incidence of the congenital rubella syndrome (CRS), when vaccine coverage is low or the natural force of infection of rubella is high. Vaccine induced herd immunity can act to the disadvantage of the unvaccinated individuals. Following earlier work we show that certain vaccination programmes could actually increase the incidence of serious cases. At first sight, this may seem inconceivable. Mathematical analysis, however, reveals that this is indeed a real possibility. Implementing a vaccination programme that is not sufficiently intensive to eradicate the infection in the population will ultimately lead to a new steady state in which the force of infection is lower than in the period before the start of the programme. As a consequence those who are not (effectively) vaccinated have a higher chance of contracting the infection at an older age. For many viral infections, the chance of serious complications is greater if the infection is acquired at an older age. Congenital rubella and measles encephalitis are striking examples (with respect to mumps the situation is less clear). When boys and girls are vaccinated around one year of age and MMR vaccine coverage is lower than approximately 50%, the total number of cases with the congenital rubella syndrome can increase in the long term. When the force of infection of rubella before implementing the vaccination programme is high - which may be the case in developing countries - the effect is more pronounced and even a relatively high vaccine uptake of 80-90% may not be sufficient to keep CRS incidence below the level prior to control.(ABSTRACT TRUNCATED AT 400 WORDS)