Human milk has been shown to be the ideal source of nutrition for most growing infants. Its composition continues to be an active area of investigation. In several studies in preterm and term infants, long-chain polyunsaturated fatty acids were found to improve the maturation of visual evoked potentials. The clinical significance of this finding, however, remains unclear. Nucleotides present in breast milk or added to infant formula seem to enhance the humoral immune response to vaccination. Whether breastfeeding protects susceptible infants from the risk of the development of diabetes mellitus type 1 is still controversial. Breastfeeding by mothers infected with the human immunodeficiency virus is not recommended. Other viruses and pollutants have also been found in breast milk. The importance of these in the long-term health of children remains to be established.