SINCE LATE December 1988, more than 160 cases of measles have been reported in the province of Quebec, Canada. Five hundred of the cases have occurred in metropolitan Montreal. In 199 (40%) of these cases, the onset of rash occurred in April (Figure 1). Detailed information is available for 486 (97%) of the 500 Montreal cases. Of these, 104 (21%) occurred in preschoolers aged 0-4 years, 328 (67% ) in school-aged persons 5-19 years of age, and 54 (11%) in adults greater than or equal to 20 years of age. Of the adults, 42 (78% ) were aged 20-29 years. Of school-aged patients, 191 (58% ) had histories of previous vaccinations. From January through March, "Operation Mise a jour" (Operation Update) was conducted in Montreal to ensure that all primary and secondary school students were adequately vaccinated against measles. Before this campaign, approximately 50,000 of the 285,000 Montreal primary and secondary school students