In 1964, the Medical Research Council undertook a trial of measles vaccine in over 36,000 United Kingdom children; 9577 of whom received live vaccine, 10,625 received inactivated followed by live vaccines, and 16,328 acted as unvaccinated controls. Participants in this study have been followed to determine the long term protection from measles vaccine and follow-up data were available on 4194, 4638 and 274 respectively. During the 5-year period 1986-90, the protective efficacy of live measles vaccine has remained high at 87%, but the 95% confidence interval was wide (-43 to 99%) due to the small numbers of cases. Between 1976 and 1990, however, the overall efficacy of the live vaccine was 92% (95% confidence interval 86 to 95%) and there was no evidence of a decline in efficacy (P = 0.13) over the 15-year period. This study suggests that the protection from live measles vaccine persists for up to 27 years after vaccination, and that no change in the current United Kingdom measles immunization policy should be made on the grounds of waning immunity.