Fat is necessary in the diets of infants and young children because of their extraordinary energy needs and limited dietary capacity. In addition, essential fatty acids provide the substrates for arachidonic acid, docosahexaenoic acid, and their metabolites. Deficiencies in the amounts of these long-chain fatty acids in the diet during infancy may affect the maturation of the central nervous system, including visual development and intelligence. Efforts to link the diet in infancy and early childhood to the development of chronic diseases in adulthood are hampered by a lack of supportive epidemiologic and clinical data. Serum cholesterol and lipid levels during childhood correlate only weakly with their levels at maturity. Studies in twins suggest that there is a large genetic component to serum lipid levels. Similarly, the correlation between obesity in early childhood and in adulthood is weak. Young children who receive fat-restricted diets in which fat accounts for 30% or less of their intake appear to grow normally but are more likely not to consume the recommended dietary allowances of many nutrients. Therefore fat should not be restricted in the diets of infants and young children. Restricting fat to approximately 30% of the calories consumed is reasonable after the age of 2 years, but the benefits of this recommendation remain to be proved.