WHO's Expanded Programme on Immunization has significantly helped to reduce global morbidity and mortality from measles. Recently, some African countries with high vaccine coverage levels have reported measles outbreaks in children above the current target age group for immunization. Outbreaks such as these are to be expected, unless close to 100% of the population are immunized with a vaccine which is 100% effective. Success of an immunization programme requires identification of the distribution and ages of susceptible children and reduction of their concentration throughout the community. Priority should be given to urban and densely populated rural areas. In large urban areas, high coverage of infants must be achieved soon after the age at which they lose their maternal antibodies and become susceptible. This will be facilitated by the introduction of high-dose measles vaccines which can be given at 6 months of age. Where measles incidence is increasing among children aged over 2 years, immunization of older children may be considered during contacts with the health care system, or at primary school entry, if this does not divert resources from immunization of younger children. Health workers should be informed of the predicted changes in measles epidemiology following immunization. The collection, analysis and use of data on measles (vaccine coverage, morbidity and mortality) should be improved at all levels of the health care system in order to monitor the immunization programme's overall impact, identify pockets of low coverage, and allow early detection of and response to measles outbreaks.