By means of peripheral computed tomography (pQCT), adult cortical bone dens
ity and volume was shown to be under a fixed rectilinear relationship regar
dless of age, sex, and presence or absence of osteoporosis. In children, ho
wever, the density-volume regression line followed a clearly different slop
e from that for adults (P < 0.001), indicating a difference in property and
composition of the cortical bone between growing bone of children and grow
nup bone of adults. Although both relative cortical volume and density incr
eased with age in both boys and girls, no significant increase of trabecula
r bone was noted in either during the growth period. When the same techniqu
e was applied to the bone of rats known to continue growing with indefinite
modeling without remodeling, the regression line between cortical bone den
sity and volume was different from that of adult humans and similar to that
of growing human children. Growing bone in a constant process of modeling
and mineralization thus seems to have cortical bone possibly with less comp
lete mineralization somewhat different from those of grown-up bone only und
ergoing remodeling. Selective cortical bone measurement by pQCT appears to
be useful in characterizing the unique properties of the cortex of the grow
ing bone.