Bone mineral content (BMC) of the midportion of the humerus was measured with the Norland model 278 bone densitometer in 31 term infants fed human milk (9 inants), cow milk-based formula (11 infants), or soy protein-based formula (11 infants) over the first year of life. Serum and urine minerals, vitamin D, 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D were measured to ensure that intake of minerals and vitamin D was not limiting. Mineralization was similar in all three groups. Over the first year, BMC had a significantly positive slope. However, BMC appeared to increase during the first 4 months, plateau at 6 months, and then resume an increasing pattern. Bone width (BW) increased steadily over the first year; therefore the BMC/BW ratio actually dropped at around 6 months. The BMC was also correlated with body length. "Average density," which was calculated by the formula BMC/pi (BW/2)2, showed a steady decline as age increased. The ratio of BMC to body length over the first year may be an option for comparing individual data to normal data when growth is abnormal. Uncorrected BMC is recommended as a measurement for the infant growing normally; attempts to calculate average density are discouraged.