During a large measles outbreak in Quebec City in 1989, two investigations conducted in parallel evaluated the relative risk of measles and measles vaccine effectiveness with respect to age at vaccination. The study was a school-based case-control study including 563 cases and 1126 classmate controls. The second was a cohort study of the siblings of school cases including 493 siblings aged between 1 and 19 years. The relative risks (RR) of measles were similar in both settings and the trend towards increased vaccine efficacy with increasing age at vaccination was highly significant (P < 0.001). Vaccine efficacy rose from 85% in children vaccinated at 12 months of age to > or = 94% in those vaccinated at 15 months and older. Even for children vaccinated at or after 18 months of age, the RR of measles was reduced when compared with children vaccinated between 15 and 17 months of age (RR 0.61, CI 95% 0.33-1.15). Small changes in the timing of initial measles vaccination can have a major impact on vaccine efficacy.