In affluent populations, breast-fed infants generally exhibit a different pattern of growth than formula-fed infants. The average weight gain of the former is lower than that of the latter, even after complementary foods are introduced. In some studies, the length gain is also lower among breast-fed infants, whereas in others there is no significant difference in linear growth between feeding groups. Growth in head circumference does not differ by feeding mode. Because of the difference in weight gain, breast-fed infants are generally leaner than formula-fed infants by 12 months of age. Breast-fed infants appear to self-regulate their energy intake at a lower level than consumed by formula-fed infants and have a lower metabolic rate. Evidence to date suggests that there are no apparent adverse consequences associated with the lower intake and slower weight gain of breast-fed infants: compared to formula-fed infants, they do not differ in activity level, and they experience less illness and appear to have enhanced cognitive development. The reasons for differences in growth patterns by feeding mode require further research.