We studied the performance of the dual-energy x-ray absorptiometry method in evaluating bone mineral, fat, and lean soft tissue mass. This method was accurate in quantifying known small amounts of calcium, lard, and lean tissue mass. It was also accurate in evaluating small animal ashed bones, fat, and muscle mass. The analytic sensitivity of the method was 40 mg for calcium, 180 mg for fat, and 270 mg for lean tissue mass. The method was highly correlated to the single-photon absorptiometry method in measuring bone mineral content at the radius bone in 32 children, r = 0.998. There was a difference between the two methods in older children. Long-term precision for a small bone phantom was 2.0%. Total-body, lumbar, and radial bone scans were performed on 14 newborn infants whose gestational ages ranged from 28 to 41 weeks. Both total-body bone mineral and fat mass increased with gestational age and weight. The infant's total-body calcium was also associated with length and lumbar and radial bone densities. The lumbar bone density was associated with birth weight, gestational age, length, body mass index, body fat, and radial bone density. Male infant's lumbar bone density to total-body calcium ratio was higher than female infant's lumbar bone density ratio. Dual-energy x-ray absorptiometry may be used in pediatrics with high accuracy, sensitivity, and precision.