Full breastfeeding (FBF) is promoted as effective for losing pregnancy weight during the postpartum period. This study evaluated whether longer FBF is associated with lower maternal postpartum weight retention (PPWR) as compared to a shorter FBF duration. The MILK (Mothers and Infants Linked for Healthy Growth) study is an ongoing prospective cohort of 370 mother-infant dyads, all of whom fully breastfed their infants for at least 1 month. Breastfeeding status was subsequently self-reported by mothers at 3 and 6 months postpartum. Maternal PPWR was calculated as maternal weight measured at 1, 3, and 6 months postpartum minus maternal prepregnancy weight. Using linear mixed effects models, by 6 months postpartum, adjusted means +/- standard errors for weight retention among mothers who fully breastfed for 1-3 (3.40 +/- 1.16 kg), 3-6 (1.41 +/- 0.69 kg), and >/=6 months (0.97 +/- 0.32 kg) were estimated. Compared to mothers who reported FBF for 1-3 months, those who reported FBF for 3-6 months and >/=6 months both had lower PPWR over the period from 1 to 6 months postpartum (p = 0.04 and p < 0.01, respectively). However, PPWR from 3 to 6 months was not significantly different among those who reported FBF for 3-6 versus >/=6 months (p > 0.05). Interventions to promote FBF past 3 months may increase the likelihood of postpartum return to prepregnancy weight.