This study examines the relationship between breast-feeding and growth from 0 to 6 and 6 to 20 mo among 185 children in a Mexican community. Infants from a previous 6-mo longitudinal study were followed up for additional anthropometric measurements at a mean age of 19.9 mo. Size at 6 mo and at follow-up were modeled as outcomes of whether infants were fully breast-fed (exclusively or predominantly breast-fed) for at least 4 mo, controlling for size at birth and 6 mo, respectively, and potential confounders. From birth to 6 mo, fully breast-fed infants had ponderal index increments of 0.07 units larger (P = 0.04) than comparison infants. There were no differences in weight. For length, an interaction between full breast-feeding and socioeconomic status (SES) was found, with fully breast-fed infants of low SES growing more than comparison infants, whereas the opposite was seen at upper SES levels. From 6 to 20 mo, fully breast-fed infants had weight and length increments of 0.53 cm (P < 0.001) and 0.72 kg (P = 0.01) smaller than those of comparison infants. For ponderal index, an interaction between mother’s education and breast-feeding revealed an inverted U-shaped response across levels of education. Additionally, logistic regressions of monthly breast-feeding on lagged measurements revealed that relatively heavier infants had higher odds of being fully breast-fed at 2 and 3 mo. Our findings indicate that the benefits of full breast-feeding on growth may be most pronounced early in life. Further research of unmeasured confounders may explain the association of full breast-feeding with slower growth beyond 6 mo.