Sir.—In 1977 the National Childhood Immunization Initiative began, with the goal to raise immunization levels to greater than 90% of all vaccine-preventable diseases in the nation's children. Although the incidence of these infections has significantly declined in the United States, a substantial number of cases continue to be reported in the 1980s.1 When the age and immunity status of affected children are known, it is clearly evident that at least half of the cases were potentially preventable. In 1982 and 1983, 68% of reported pertussis cases in children 3 months to 6 years old did not have adequate immunization with diphtheria-tetanuspertussis (DTP) vaccine.2 In addition, 20.6% of all cases of measles in the first half of 1985 occurred in children 15 months to 4 years of age, and 69.2% of measles cases in preschool-age children were preventable.3 While specific laws require proof of adequate immunity for