Recent reports indicate that breastfeeding rates continue to be dramatically lower among WIC participants, compared with other US mothers. The WIC Infant Feeding Practices Study was a nationally representative 1-year longitudinal study of WIC participants that obtained information about attitudes regarding infant feeding and about infant-feeding practices. Hispanic mothers were most likely to agree with statements about benefits of breastfeeding, and Black mothers were most likely to agree with statements about barriers. Concern about insufficient milk was common in all ethnic groups. Perceived benefits were associated with breastfeeding initiation (P < .05), longer breastfeeding duration (P < .01), and later formula initiation (P < .01); for barriers, the opposite pattern was found. Breastfeeding mothers who reported concern about insufficient milk breastfed for shorter durations (P < .001) and initiated formula earlier (P < .01). These results suggest possible messages that should be communicated as part of a re-energized WIC breastfeeding promotion campaign. In particular, maternal anxiety about insufficient breast milk must be addressed. J Hum Lact. 23(4):314-324.