An epidemic of between 100,000 and 200,000 cases of measles during 1995 has been predicted in England and Wales. This prediction was based on epidemiological evidence from several sources. Notifications of measles to the Office of Population Censuses and Surveys have risen in 1994, with a high proportion of cases in children aged over 10 years. An increase in the incidence of measles was seen in data from other sources, including laboratory reports of confirmed infections and consultations with general practitioners for new episodes of measles. Antibody tests were performed on saliva and serum from notified cases in several districts. Over three quarters of the notified cases in 1994 that were confirmed occurred in children of school age. The proportion of children aged 7 to 14 years who were susceptible to measles, obtained from studies of the age specific prevalence of antibody, rose from 6.0% (146/2453) in 1986 and 1987 to 9.2% (144/1565) in 1991. Mathematical modelling has predicted that the level of susceptibility anticipated in the school age population in 1995 would have been sufficient to allow a resurgence of measles. Over half of the cases in the resulting epidemic would have occurred in people aged at least 10 years and, because mortality is higher in this older age group, between 30 and 60 deaths would have occurred. A mass campaign to immunise all children of school age is expected to cause an immediate reduction in disease transmission and prevent a substantial toll of morbidity and mortality.