A measles outbreak in an inner-city area primarily involved preschool-aged children younger than 5 years of age. The reasons why 31 unvaccinated preschool children with measles disease had not been vaccinated were investigated. For some patients, health care providers missed opportunities to vaccinate eligible patients against measles. Of the 26 patients whose full immunization status was known, ten (38%) were vaccinated with diphtheria and tetanus toxoids and pertussis vaccine and/or oral poliovirus vaccine at a time when they could have received measles vaccine simultaneously, according to recommendations of the Immunization Practices Advisory Committee and the American Academy of Pediatrics. In addition, five of ten health care providers interviewed missed at least one opportunity to administer measles vaccine because of a minor illness that was not a contraindication to vaccination. Unvaccinated patients were more likely to receive health care in the public sector, have single mothers, and have parents who had no knowledge of existing vaccines; they were less likely to be age-appropriately immunized with other antigens. If measles immunization levels among preschool children in the United States are to be increased, education of both health care providers and parents, coupled with innovative strategies targeted to preschool children, particularly of low socioeconomic groups in inner cities, are needed.