BACKGROUND: Vaccination-promoting strategies in the Supplemental Nutrition Program for Women, Infants, and Children (WIC) have been shown to produce dramatic improvements in coverage and other health outcomes. OBJECTIVES: To determine national and state-specific population-based vaccine coverage rates among preschool children who participate in the WIC program, and to describe the strategies for promoting vaccination in WIC. DESIGN/METHODS: Demographic data, WIC participation, and vaccination histories for children aged 24 to 35 months in 1999 were collected from parents through the National Immunization Survey. The healthcare providers for the children in the survey were contacted to verify and complete vaccination information. We defined children as up-to-date (UTD) if they had received four doses of diphtheria and tetanus toxoids and pertussis vaccine (DPT), three doses of poliovirus vaccine, one dose of measles-mumps-rubella vaccine (MMR), and three doses of Haemophilus influenzae type b vaccine (Hib) by 24 months. Description of state-level vaccination-promoting activities in WIC was collected through an annual survey completed by the state WIC and immunization program directors. RESULTS: Complete data were collected on 15,766 children, of whom 7783 (49%) participated in WIC sometime in their lives. Nationally, children who had ever participated in WIC were less well-immunized at 24 months compared to children who had not: 72.9% UTD (95% CI, 71.3-74.5) versus 80.8% UTD (95% CI, 79.5-82.1), respectively. In 42 states, 24-month coverage among WIC participants was less than among non-WIC participants, including 13 states where the difference was > or = 10%. Vaccination activities linked with WIC were reported from 76% of 8287 WIC sites nationwide. States conducting more-frequent interventions and reaching a higher proportion of WIC participants had 40% higher vaccination coverage levels for the WIC participants in that state (p<0.05). CONCLUSIONS: Children served by WIC remain less well-immunized than the nation's more-affluent children who do not participate in WIC. Thus, WIC remains a good place to target these children. This study provides evidence that fully implemented WIC linkage works to improve vaccination rates. Strategies that have been shown to improve the vaccination coverage levels of WIC participants should be expanded and adequately funded to protect these children.