Extensive ethnic conflict within the Kosovo region of the Federal Republic of Yugoslavia and an organized bombing campaign by the North Atlantic Treaty Organization led to mass population displacement in 1998 and early 1999. In April 1999, approximately 500,000 Kosovar Albanians fled into the Yugoslavian Republic of Montenegro and the neighboring countries of Albania, Bosnia-Herzegovina, and the Former Yugoslav Republic of Macedonia (FYROM). Of the estimated 130,000 refugees who fled to FYROM, approximately 65,000 were housed in seven refugee camps. A major public health concern in these camps was the prevention of vaccine-preventable diseases, particularly measles. In response, the FYROM Ministry of Health (MOH) in collaboration with the United Nations Children's Fund (UNICEF) and International Medical Corps, a nongovernmental organization, planned and implemented a mass vaccination campaign. This report describes the first campaign (April 26-May 10, 1999), its results, and follow-up activities.