This study was designed to estimate energy intake in exclusively formula-fed infants. Formula intake of twenty-four 1- and 4-month-old infants was studied for 5 consecutive days; six boys and six girls were in each age group. Intake was estimated by laboratory-determined weights of formula consumed, spilled, and regurgitated. Two additional methods were used to estimate intake in the first nine infants during the 1st day of observation: test-weighing the infant at each feeding and mother's weighing of formula consumed, regurgitated, and spilled at each feeding. No consistent differences were detected among methods, but test-weighing appeared to have the greatest feed-to-feed variability. Intake was estimated to be 747 +/- 100 g or 125.5 +/- 17 kcal/kg, and 958 +/- 131 g or 94.0 +/- 13 kcal/kg for 1- and 4-month-old infants, respectively. The day-to-day variability (expressed as the coefficient of variation) was 13 and 15% (CV, g/kg) for 1- and 4-month-old infants, respectively. Between-infant variability of intake was approximately 8% (CV, g/kg) for both age groups. Energy intakes of 1-month-old formula-fed infants were similar to published values of breast-fed infants of similar age, but the energy intakes of 4-month-old formula-fed infants were significantly higher than values published for 4-month-old breast-fed infants.