Context.Each year 2.1 million children 19 to 35 months of age are undervaccinated.Among these are children who have received no vaccinations.Unvaccinated children are at increased risk of acquiring and transmitting vaccine-preventable diseases.Objectives.To assess whether the characteristics of children with no vaccinations differ from those of undervaccinated children, tomonitor trends in the numbers of unvaccinated children, and to identify states with high rates and counties with large numbers of unvaccinated children.Design.A nationally representative probability sample of children 19 to 35 months of age was collected annually between 1995 and 2001.Vaccination histories were ascertained from children’s medical providers.Undervaccinated children had received ≥1 dose of diphtheriatetanus-pertussis, polio, measles, Haemophilus influenzae type b, hepatitis B, or varicella vaccine butwere not fully vaccinated.Unvaccinated children were children who were reported as having no medical providers and having received no vaccinations or children whose medical providers reported administering no vaccinations.Participants.A total of 151 720 children sampled between 1995 and 2001, 795 of whom were unvaccinated.Results.Undervaccinated children tended to be black, to have a younger mother who was not married and did not have a college degree, to live in a household near the poverty level, and to live in a central city.Unvaccinated children tended to be white, to have a mother who was married and had a college degree, to live in a household with an annual income exceeding ＄75 000, and to have parents who expressed concerns regarding the safety of vaccines and indicated that medical doctors have little influence over vaccination decisions for their children.Unvaccinated children were more likely to be male than female.Annually, ～17 000 children were unvaccinated.The largest numbers of unvaccinated children lived in counties in California, Illinois, New York, Washington, Pennsylvania, Texas, Oklahoma, Colorado, Utah, and Michigan.States that allowed philosophical exemptions to laws mandating vaccinations for children as they entered school had significantly higher estimated rates of unvaccinated children.Conclusions.Unvaccinated children have characteristics that are distinctly different from those of undervaccinated children.Unvaccinated children are clustered geographically, increasing the risk of transmitting vaccine-preventable diseases to both unvaccinated and undervaccinated children. Context.Each year 2.1 million children 19 to 35 months of age are undervaccinated. Among these are children who have received no vaccinations. Unvaccinated children are at increased risk of acquiring and transmitting vaccine-preventable diseases. ObjectivesTo assess the the characteristics of children with no vaccinations differ from those of undervaccinated children, tomonitor trends in the numbers of unvaccinated children, and to identify states with high rates and counties with large numbers of unvaccinated children. Design. A nationally representative probability sample of children 19 to 35 months of age was collected annually between 1995 and 2001. Vaccination histories were ascertained from children’s medical providers. Undervaccinated children had received ≥ 1 dose of diphtheriatetanus-pertussis, polio, measles, Haemophilus influenzae type b, hepatitis B, or varicella vaccine but were not fully vaccinated. Unvaccinated children were children who were reported as having no medical p roviders and having received no vaccinations or children whose medical providers reported in no. vaccinations. Participants. A total of 151 720 children sampled between 1995 and 2001, 795 of whom were unvaccinated. Results. Undervaccinated children ten to to black, to have a younger mother who was not married and did not have a college degree, to live in a household near the poverty level, and to live in a central city .Unvaccinated children tended to be white, to have a mother who was married and had a college degree, to live in a household with an annual income exceeds $ 75 000, and to have parents who published opinions regarding the safety of vaccines and indicated that medical doctors have little influence over vaccination decisions for their children .Unvaccinated children were more likely to be male than Anonymous, ~ 17 000 children were unvaccinated. The largest numbers of unvaccinated children lived in counties in California, Illinois, New York, Washington, Pen. nsylvania, Texas, Oklahoma, Colorado, Utah, and Michigan. States that allowed philosophical exemptions to laws mandating vaccinations for children as they enrolled schools had significantly higher rates of estimates than unvaccinated children. Conclusions. Unvaccinated children have characteristics that are distinctly different from those of Undervaccinated children. Unvaccinated children are clustered geographically, increasing the risk of transmitting vaccine-preventable diseases to both unvaccinated and undervaccinated children.