There is limited information on the validity of bone mineral content measurement by dual energy X-ray absorptiometry (DXA BMC) for use in subjects with low body mass. We evaluated the accuracy and precision of DXA in piglets (body weight 886–5526 g, median 2096 g). Stepwise multiple regression analyses showed that ash weight is the major determinant of DXA BMC (adjusted r2 = 0.98, RMS residual = 3.61 g). The intercept was not significantly different from zero. DXA BMC measurements of other piglets under various clinical situations showed no significant effect from the use of cotton blanket, diaper, or positioning (prone, supine, lateral). In vivo replication of DXA BMC measurements of infants at a postnatal age of from 1–350 days showed a slope of 0.99 and high correlation coefficient (r2 = 0.99, RMS residual = 3.59 g). The intercept was not significantly different from zero, and the average coefficient of variation of duplicate DXA BMC in infants was 2.8%. We conclude that DXA BMC reliably but proportionately underestimates ash weight and is a highly precise method for measuring bone mineral status in young pediatric subjects.