Context. The Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) is the largest food supplement program in the United States, serving almost 7 500 000 participants in 2002. Because the program is a grant program, rather than an entitlement program, Congress is not mandated to allocate funds to serve all eligible participants. Little is known about the effects of WIC on infant growth, health, and food security. Objective. To examine associations between WIC participation and indicators of underweight, overweight, length, caregiver-perceived health, and household food security among infants ≤12 months of age, at 6 urban hospitals and clinics. Design and Setting. A multisite study with cross-sectional surveys administered at urban medical centers in 5 states and Washington, DC, from August 1998 though December 2001. Participants. A total of 5923 WIC-eligible caregivers of infants ≤12 months of age were interviewed at hospital clinics and emergency departments. Main Outcome Measures. Weight-for-age, length-for-age, weight-for-length, caregiver’s perception of infant’s health, and household food security. Results. Ninety-one percent of WIC-eligible families were receiving WIC assistance. Of the eligible families not receiving WIC assistance, 64% reported access problems and 36% denied a need for WIC. The weight and length of WIC assistance recipients, adjusted for age and gender, were consistent with national normative values. With control for potential confounding family variables (site, housing subsidy, employment status, education, and receipt of food stamps or Temporary Assistance for Needy Families) and infant variables (race/ethnicity, birth weight, months breastfed, and age), infants who did not receive WIC assistance because of access problems were more likely to be underweight (weight-for-age z score = −0.23 vs 0.009), short (length-for-age z score = −0.23 vs −0.02), and perceived as having fair or poor health (adjusted odds ratio: 1.92; 95% confidence interval: 1.29–2.87), compared with WIC assistance recipients. Rates of overweight, based on weight-for-length of >95th percentile, varied from 7% to 9% and did not differ among the 3 groups but were higher than the 5% expected from national growth charts. Rates of food insecurity were consistent with national data for minority households with children. Families that did not receive WIC assistance because of access problems had higher rates of food insecurity (28%) than did WIC participants (23%), although differences were not significant after covariate control. Caregivers who did not perceive a need for WIC services had more economic and personal resources than did WIC participants and were less likely to be food-insecure, but there were no differences in infants’ weight-for-age, perceived health, or overweight between families that did not perceive a need for WIC services and those that received WIC assistance. Conclusions. Infants ≤12 months of age benefit from WIC participation. Health care providers should promote WIC utilization for eligible families and advocate that WIC receive support to reduce waiting lists and eliminate barriers that interfere with access.