The rubella-specific antibody levels of children vaccinated with RA 27/3 rubella vaccine have been determined over the 15 years since vaccination. Over the period monitored, titres have declined at a comparable rate to those observed in children who had experienced natural rubella infection. In both cohorts the mean rate of decay was similar throughout the 15 years of the study. One in eleven vaccinated children monitored for the entire period of the study reverted to a state of susceptibility to rubella as judged by routine rubella antibody tests used in practice today. The implications of the findings for rubella prophylaxis are discussed.