OBJECTIVEThe impact of exclusive breastfeeding (EBF) on child body mass index (BMI) from birth through 60 months of age was investigated.METHODS60,190 mothers and children attending Women, Infant and Children (WIC) clinics in the central coast counties of California provided data on breastfeeding duration, maternal pre‐pregnancy BMI, and child weight‐for‐length (WL) were measured at 3 month intervals until 5 years of age. Missing time points were interpolated in determining child BMI/WL z‐score trajectories plotted on the Center of Disease Control infant and child growth curves. Multivariate analysis of variance was used to contrast the impact of not EBF, EBF from 0‐3 months, and EBF from 0‐6 months in Hispanic (n=43,360) versus Other (n=16,830) children, controlling for maternal pre‐pregnancy BMI.RESULTSDuration of EBF from 0‐3 and 0‐6 months of age was protective against child BMI z‐scores being above the average growth trajectory (P=0.0001). A significant inverse association between EBF and overall BMI/WL z‐scores for both males and females (P<0.0001) was found. Impact of EBF on child growth trajectory began to weaken at 15 months for females and 18 months for males. Children who were not EBF had higher mean BMI/WL z‐scores at every time point than those who were EBF. Those who were EBF for 0‐3 months had a lower BMI; those who were EBF for 0‐6 months had even lower BMI z‐scores by 5 years of age. Hispanic children had a higher BMI growth curve than non‐Hispanic children even after adjusting for EBF and pre‐pregnancy BMI. Mothers with a higher pre‐pregnancy BMI tended to have children with higher BMI/WL z‐scores (P<0.0001).CONCLUSIONThe study supports public health efforts to encourage EBF for at least 3 months and optimally for 6 months.