The pediatrician is faced with a difficult challenge in providing recommendations for optimal nutrition in older infants. Because the milk (or formula) portion of the diet represents 35% to 100% of total daily calories and because WCM and breast milk or infant formula differ markedly in composition, the selection of a milk or formula has a great impact on nutrient intake. Infants fed WCM have low intakes of iron, linoleic acid, and vitamin E, and excessive intakes of sodium, potassium, and protein, illustrating the poor nutritional compatibility of solid foods and WCM. These nutrient intakes are not optimal and may result in altered nutritional status, with the most dramatic effect on iron status. Infants fed iron-fortified formula or breast milk for the first 12 months of life generally maintain normal iron status. No studies have concluded that the introduction of WCM into the diet at 6 months of age produces adequate iron status in later infancy; however, recent studies have demonstrated that iron status is significantly impaired when WCM is introduced into the diet of 6-month-old infants. Data from studies abroad of highly iron-deficient infant populations suggest that infants fed partially modified milk formulas with supplemental iron in a highly bioavailable form (ferrous sulfate) may maintain adequate iron status. However, these studies do not address the overall nutritional adequacy of the infant's diet. Such formulas have not been studied in the United States. Optimal nutrition of the infant involves selecting the appropriate milk source and eventually introducing infant solid foods.(ABSTRACT TRUNCATED AT 250 WORDS)