• We tested the best anatomic site, reliability, and reproducibility of singlephoton absorptiometric bone density measurements in premature and term newborns. Humerus and radius measurements were compared using a commercially available densitometer. The humerus was a more reliable site of measurement, particularly in the very-low-birth-weight infant. Normal ranges of humerus bone mineral content (BMC) were defined for infants of 24 to 42 weeks' gestational age at birth. Humerus BMC correlated with gestational age and birth weight of patients. We conclude that bone densitometer measurements can be successfully performed, even in very-low-birth-weight infants, when the humerus is used as the measurement site. We define normal humerus BMC values for use in diagnosis and evaluation of efficacy of treatment in infants who are at higher risk for osteopenia of prematurity.