The concept of exclusive breastfeeding in relation to daily feeding practices and to health and growth of infants in an affluent society was examined. In a descriptive longitudinal prospective study 506 mother-infant pairs were followed from birth through the greater part of the first year. Feeding was recorded daily, and health and growth were recorded fortnightly.

Large individual variations were seen in breastfeeding patterns. A wide discrepancy between the exclusive breastfeeding rates obtained from "current status" data and data "since birth" was found.

Using a strict definition of exclusive breastfeeding from birth and taking into account the reasons for giving complementary feeding, the study showed that many exclusively breastfed infants had infections early in life, the incidence of which increased with age, despite continuation of exclusive breastfeeding. However, truly exclusively breastfed infants seem less likely to suffer infections than infants who receive formula in addition to breast milk. Increasing formula use was associated with an increasing likelihood of suffering respiratory illnesses. The growth of exclusively breastfed infants was similar to that of infants who were not exclusively breastfed.

The health of newborn infants during the first year of life was associated with factors other than feeding practices alone. Some of these factors may be prenatal, since increasing birth weight was associated with an increasing likelihood of having respiratory symptoms, even in exclusively breastfed infants. However, exclusive breastfeeding was shown to be beneficial for the health of the infant even in an affluent society.