During a measles outbreak in Northern Ireland (between October 1988 and March 1989) it was noted that a proportion of cases had occurred in children who had previously been vaccinated against measles. A study was, therefore, set up to provide a rapid estimate of vaccine efficacy. Vaccine efficacy was calculated to be 94% (95% confidence limits, 91% to 96%). Investigation of the computerised records at 31 December 1991 revealed that 94% of children in the study cohort had received measles vaccine. As vaccination coverage increases, a higher proportion of cases of measles will, inevitably, have a history of vaccination.