Inner-city infants (n = 565) enrolled in the WIC program were randomly assigned at 6 months of age to either of 2 groups: (1) voucher incentive (frequency of issuance of food vouchers based on immunization status) plus reminder-recall (calls and/or letters to families of under-vaccinated children) or (2) voucher incentive alone. At 12 months, both groups' immunization levels were high and not significantly different: 80% +/- 4% versus 79% +/- 5% (P =.749).