The feeding of cow's milk has adverse effects on iron nutrition in infants and young children. Several different mechanisms have been identified that may act synergistically. Probably most important is the low iron content of cow's milk. It makes it difficult for the infant to obtain the amounts of iron needed for growth. A second mechanism is the occult intestinal blood loss, which occurs in about 40% of normal infants during feeding of cow's milk. Loss of iron in the form of blood diminishes with age and ceases after 1 year of age. A third factor is calcium and casein provided by cow's milk in high amounts. Calcium and casein both inhibit the absorption of dietary nonheme iron. Infants fed cow's milk receive much more protein and minerals than they need. The excess has to be excreted in the urine. The high renal solute load leads to higher urine concentration during the feeding of cow's milk than during the feeding of breast milk or formula. When fluid intakes are low and/or when extrarenal water losses are high, the renal concentrating ability of infants may be insufficient for maintaining water balance in the face of high water use for excretion of the high renal solute. The resulting negative water balance, if prolonged, can lead to serious dehydration. There is strong epidemiological evidence that the feeding of cow's milk or formulas with similarly high potential renal solute load places infants at an increased risk of serious dehydration. The feeding of cow's milk to infants is undesirable because of cow's milk's propensity to lead to iron deficiency and because it unduly increases the risk of severe dehydration.