OBJECTIVE To determine whether the ad libitum addition of solid foods to the diet of exclusively human milk-fed infants will increase energy intake and reverse the decline in weight-for-age percentiles observed during the exclusive breast-feeding period. METHODS Weekly or biweekly measures of growth were made longitudinally on a cohort of infants from birth to 36 weeks of age, and monthly measures of nutrient intake were made from 16 weeks of age until 10 weeks after solid foods were introduced into the diet. METHODS Volunteer mother-infant pairs from middle and upper income groups who met entry criteria, including the intention to breast-feed exclusively for at least for 16 weeks; 58 pairs entered and 45 pairs completed the study. METHODS Solid foods were introduced at a time determined by the mother and the pediatrician; solid foods from controlled lot numbers were provided for each infant. RESULTS After solid foods were added, daily human milk intake declined at a rate of 77 gm/mo (p less than 0.001). Milk composition did not change during the observation period. Daily total energy intake increased 29 kcal/mo, but no changes were noted in energy intake when consumption was normalized for body weight. Weight (National Center for Health Statistics percentiles) at 28 weeks was 13 percentiles lower than that at birth, and length at 28 weeks was 1 percentile lower than at week 1. Weight and length percentiles at 28 weeks, when compared with peak values at 8 weeks, had dropped 19 and 14 percentiles, respectively. CONCLUSIONS Energy intake of human milk-fed infants did not increase after solid foods were added to their diet but was maintained at approximately 20% below recommended levels. Energy intake appeared to reflect infant demands. These data suggest that the recommendations for the energy requirements of infancy should be reevaluated. The growth pattern of exclusively breast-fed infants differs from that of the National Center for Health Statistics reference population. These observations raise questions about the adaptive response of human milk-fed infants to different levels of energy intake and about the estimations of energy requirements based on the sum of basal metabolism, activity, growth, and diet-induced thermogenesis.