Assessment of skeletal development using a nonionizing method would be desirable in critically ill preterm infants. We investigated the second metacarpus by quantitative ultrasound (QUS). Cross-sectional data were collected in 132 preterm or term infants measured within 24 h from birth and in 142 term infants up to the age of 18 mo. Longitudinal data were collected in 150 preterm infants up to the age of 14 mo. Cross-sectional data were used to devise reference curves for metacarpal speed of sound (mcSOS) and metacarpal bone transmission time (mcBTT). Both parameters increase during the last trimester of gestation. After birth, mcSOS declines up to the 6 mo and then increases up to 18 mo of life. McBTT values remain stable after birth. At birth, QUS values of preterm infants are lower than those observed at birth in term infants. In the longitudinal study, mcSOS showed a trend similar to that observed among term infants, nevertheless, values are lower up to 4 -6 mo of life. Among preterm infants, mcBTT increases until it reaches values observed in term infants. Preterm infants in their first months of life have lower QUS values compared with term infants of same weight or length. This study demonstrates that it is possible to follow skeletal development and maturation by QUS in preterm infants. More specifically, the mcBTT values may provide information on bone tissue that is independent of length and weight of the preterm infant. The method here described is safe, repeatable, and reliable. The incidence of congenital disorders of skeletal development is estimated to be about 1 in 4000 births (1). Most of them are clinically apparent at birth. Numerically more prominent are neonates suffering from intrauterine growth retardation. Further, in infants born before 28 wk of gestation and/or weighing less than about 800 g at a few weeks after birth, a temporary postnatal scarcity of bone tissue may appear. This is called osteopenia of prematurity, and may lead to spontaneous fractures (2,3). Presently, DXA is the most extensively used method for the assessment of bone mineralization (4). DXA has been used to measure bone mineral content in newborns (5-7). In longitudinal studies, DXA suffers from important limitations such as cumulative radiation dose and motion artefacts; a further limitation to the use of DXA is represented by the fact that mineral accretion and bone growth are not parallel (3). In these infants, the assessment of skeletal development at birth and in early infancy using a nonirradiating method would be desirable.QUS technique has been used for the past 10 y to assess bone tissue in postmenopausal osteoporosis (8 -11). Recent studies have shown that ultrasound can be utilized to assess bone mineral disorders in children and neonates. In preterm and term newborns, radius, humerus, and tibia were used as sites of measurement (12-16). Bearing in mind the special caution demanded by critically ill and very small infants, we sought an ultrasound method and site of measurement to achieve reliable and repe...