Objective: To explore how sources of familial encouragement are associated with breast-feeding initiation and duration among a national sample participating in the US Special Supplemental Nutrition Program for Women, Infants, and Children (WIC). Design: This study uses the 2013–2015 WIC Infant and Toddler Feeding Practices Study 2 (WIC ITFPS-2) data. Breast-feeding initiation was measured at the first month, while duration was derived from a composite of the first 13 months. The analysis used logistic and linear regression to explore the association between encouragement sources and breast-feeding outcomes. Setting: A nationally representative sample of WIC participants in the USA. Participants: WIC participants who completed the 13-month interview of the WIC ITFPS-2 (n 2807). Results: Encouragement was significantly associated with both initiation and duration. Each source of encouragement was associated with a 3·2 (95 % CI 2·8, 3·8) increase in odds of initiating breast-feeding in the unadjusted model and 3·0 (95 % CI 2·5, 3·6) increased odds, controlling for age, education, nativity, poverty status, race and ethnicity (<0·0001). When predicting log duration, each percent increase in source of encouragement was associated with an increasing duration on average by 0·003 d (95 % CI 0·2, 0·3, <0·0001). When controls were added, it was associated with an increase of an average of 0·002 d (95 % CI 0·2, 0·3) per percent increase in encouragement source (<0·0001). Conclusions: Women who receive encouragement appear to be more likely to breastfeed. Additional work is needed to explore sources of encouragement and how to include them in intervention work.