Measurements were made on hand radiographs of normal girls and patients with Turner's syndrome. Normal girls going through puberty display increases in hand bone dimensions which are highly correlated with height and bone age. We report such changes in length and total bone width of metacarpal II (MCII) and proximal phalanx II (PPII), and in combined cortical thickness and cortical area of MCII. For the relative cortical area, the correlations are less pronounced. In comparison with controls of similar height, MCII of patients with Turner's syndrome has greater length and total bone width and combined cortical thickness. Turner patients treated with sex steroids undergo skeletal maturation as well as a rise in MCII relative cortical area (a measure of bone density), but fail to show a commensurate increase in MCII length and in MCII cortical area (a measure of bone mass). It is concluded that in untreated Turner patients, bone density as well as bone mass are normal for the (retarded) bone age, and that sex steroids which promote skeletal maturity likewise raise bone density but not bone mass. The (relative) insensitivity of bone mass and the lack of accelerated statural growth in response to administered sex steroids may be inborn characteristics of the Turner skeleton.