• We evaluated epidemiologic features of 54 patients with measles seen at Wyler Children's Hospital during a 1983 measles epidemic in Chicago. Forty-one (76%) were less than 5 years old, and 23 (43%) were less than 16 months old. We found evidence of a low rate of measles immunity in preschool-aged children of immunization age immediately before the epidemic; only 62% of 173 emergency room patients this age had antibody on enzyme-linked immunosorbent assay. These observations, together with failure of school-based control measures to significantly affect the observed course of the epidemic, indicate that the preschool population sustained the epidemic. Unimmunized preschool-aged children are important in the current epidemiology of measles, and improved immunization strategies are needed to control measles in this population in Chicago and perhaps other cities.