Objective: Pregnancy-related weight retention can contribute to obesity, and breast-feeding may facilitate postpartum weight loss. We investigated the effect of breast-feeding on postpartum weight retention. Design: A retrospective follow-up study of weight retention, compared in women who were fully breast-feeding, combining breast-feeding with formula-feeding (mixed feeding), or formula-feeding at 3 months (n 14 330) or 6 months (n 4922) postpartum, controlling for demographic and weight-related covariates using multiple linear regression. Setting: The North Carolina Special Supplemental Nutrition Program for Women, Infants, and Children (WIC). Subjects: Participants in the North Carolina WIC Programme who delivered a baby between 1996 and 2004. Results: In covariate-adjusted analyses, there was no association between breastfeeding and weight retention at 3 months postpartum. At 6 months postpartum, as compared to formula-feeders, mean weight retention was 0?84 kg lower in mixed feeders (95 % CI 0?39, 1?29; P 5 0?0002) and 1?38 kg lower in full breast-feeders (95 % CI 0?89, 1?87; P # 0?0001). Conclusions: Breast-feeding was inversely associated with weight retention at 6 months postpartum in this large, racially diverse sample of low-income women. Further, full breast-feeding had a larger protective effect than did breast-feeding combined with formula-feeding.