With the use of measles vaccine since 1967, Hong Kong has experienced a low incidence of measles until a major outbreak in 1988. A shift in the distribution of susceptible children to older age groups was suddenly accelerated in the 1988 outbreak. The attack rate increased by 18.9-fold for children greater than 10 years old, while that for those in the best-protected age group of 1-4 years was only 2.2-fold. Of the cases during that outbreak, 56.3% would have been considered preventable with the present vaccination regimen, and vaccine failures accounted for only 20.4% of the cases. Present control strategies aim at increasing the coverage rate rather than introducing a two-dose regimen, which may be necessary when vaccine failures account for a larger proportion of measles cases.