OBJECTIVE: The US Surgeon General has recommended that peer counseling to support breastfeeding become a core service of the Supplemental Nutrition Program for Women, Infants, and Children (WIC). As of 2008, 50% of WIC clients received services from local WIC agencies that offered peer counseling. Little is known about the effectiveness of these peer counseling programs. Randomized controlled trials of peer counseling interventions among low-income women in the United States showed increases in breastfeeding initiation and duration, but it is doubtful that the level of support provided could be scaled up to service WIC participants nationally. We tested whether a telephone peer counseling program among WIC participants could increase breastfeeding initiation, duration, and exclusivity. METHODS: We randomly assigned 1948 WIC clients recruited during pregnancy who intended to breastfeed or were considering breastfeeding to 3 study arms: no peer counseling, 4 telephone contacts, or 8 telephone contacts. RESULTS: We combined 2 treatment arms because there was no difference in the distribution of peer contacts. Nonexclusive breastfeeding duration was greater at 3 months postpartum for all women in the treatment group (adjusted relative risk: 1.22; 95% confidence interval [CI]: 1.10–1.34) but greater at 6 months for Spanish-speaking clients only (adjusted relative risk: 1.29; 95% CI: 1.10–1.51). The likelihood of exclusive breastfeeding cessation was less among Spanish-speaking clients (adjusted odds ratio: 0.78; 95% CI: 0.68–0.89). CONCLUSIONS: A telephone peer counseling program achieved gains in nonexclusive breastfeeding but modest improvements in exclusive breastfeeding were limited to Spanish- speaking women.