Responding to the declaration of regional polio eradication in 2000, Western Pacific Region (WPR) has been strengthening the efforts for measles elimination. Nevertheless, we questioned the feasibility of the measles elimination in Laos, where measles was still endemic. This study was undertaken with the aim to examine the impact of mass measles vaccination campaign, by comparing the prevalence of measles antibodies between before and after the campaign in 2000, among the children aged 9 months to 4 years in the two pilot sites. The constraints of mass vaccination campaign have been explored in this study. It was also suggested that more efforts should be put on reconstruction of the existing routine immunization service in Laos, which had been weakened by aggressive eradication programme. (C) 2002 Elsevier Science Ltd. All rights reserved.