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To examine the risk factors for measles vaccine failure and to evaluate the effectiveness of a selective revaccination strategy during a measles outbreak.Matched case-control study.Thirty-one schools in Mississauga, Ont.Eighty-seven previously vaccinated school-aged children with measles that met the Advisory Committee on Epidemiology's clinical case definition for measles. Two previously vaccinated control subjects were randomly selected for each case subject from the same homeroom class.All susceptible contacts were vaccinated, and contacts who had been vaccinated before Jan. 1, 1980, were revaccinated. When two or more cases occurred in a school all children vaccinated before 1980 were revaccinated.Risk of measles associated with age at vaccination, time since vaccination, vaccination before 1980 and revaccination.Subjects vaccinated before 12 months of age were at greater risk of measles than those vaccinated later (adjusted odds ratio (OR) 7.7, 95% confidence interval (CI) 1.6 to 38.3; p = 0.01). Those vaccinated between 12 and 14 months of age were at no greater risk than those vaccinated at 15 months or over. Subjects vaccinated before 1980 were at greater risk than those vaccinated after 1980 (adjusted OR 14.5, 95% CI 1.5 to 135.6). Time since vaccination was not a risk factor. Revaccination was effective in reducing the risk of measles in both subjects vaccinated before 1980 and those vaccinated after 1980 (adjusted OR reduced to 0.6 (95% CI 0.1 to 5.3) and 0.3 (95% CI 0.13 to 2.6) respectively). However, only 18 cases were estimated to have been prevented by this strategy.Adherence to routine measles vaccination for all eligible children is important in ensuring appropriate coverage with a single dose. The selective revaccination strategy's high labour intensiveness and low measles prevention rate during the outbreak bring into question the effectiveness of such a strategy.