The aim of this study was to examine the association of breastfeeding practices with the growth trajectories of children's cognitive development. We used data from the Child Development Supplement (CDS) of the Panel Study of Income Dynamics (PSID) with variables on presence and duration of breastfeeding and standardized test scores obtained during three different panel waves (N = 2681). After adjusting for covariates we found that breastfed children had higher test scores but that breastfed and non‐breastfed children had similar growth trajectories in test scores over time. The results indicate that breastfeeding has an important association with test scores, and that subsequent schooling and other experiences during adolescence do not eliminate the breastfeeding gap that appears in very early childhood. The aim of this study was to examine the association of breastfeeding practices with the growth trajectories of children's cognitive development. We used data from the Child Development Supplement (CDS) of the Panel Study of Income Dynamics (PSID) with variables on presence and duration of breastfeeding and standardized test scores obtained during three different panel waves (N = 2681). After adjusting for covariates we found that breastfed children had higher test scores but that breastfed and non‐breastfed children had similar growth trajectories in test scores over time.