The present article illustrates the extent of secondary vaccine failure after vaccination for measles, mumps and rubella (MMR). Secondary vaccine failure means loss of the immunity induced by vaccination to such an extent that infection becomes possible. Serological investigations carried out with follow-up periods of up to 16 years after vaccination for measles, 21 years after vaccination for rubella and 12 years after vaccination for mumps reveal that loss of antibodies occurs with the elapse of time but that the clinical significance of this is probably very limited. Where all three types of vaccination are concerned, secondary vaccine failure has hitherto been very seldom. Infection with measles after secondary vaccine failure is generally described as running a milder course. In rare cases, rubella re-infection has resulted in infection in utero, so that a slight risk of congenital rubella cannot be entirely excluded after successful vaccination. No extensive systematic investigations of the effect of revaccination have been carried out and, similarly, the optimal interval between two or more vaccinations has not been illustrated in more detail in the literature. Subclinical infection is not uncommon after all three vaccines. Where measles is concerned, immunity may possibly be regarded as a continuum which, depending upon the antibody level, protects the individual from various degrees of clinical disease. If wild virus can be spread via individuals with subclinical infections, it is doubtful whether population immunity (herd immunity), which is necessary to eliminate the three diseases, can be attained in large populations.(ABSTRACT TRUNCATED AT 250 WORDS)