125I Computed Tomography (CT) allows for the selective determination of trabecular and compact bone mineral parameters in the radius. Using this technique the effects of high dose oestrogen treatment in 11 tall girls, and of high dose testosterone treatment in 5 tall boys were monitored. In both groups trabecular bone density (TBD) increased steadily during treatment at a rate of about 1% per month. Also in both groups the compact bone mineral increased steadily. These results are compared with those from a cross sectional study on 49 normal children and 36 normal adults, in whom TBD was found to be independent of age and sex, so that the increases in TBD in both treatment groups can be attributed directly to the influence of the sex hormones. Since the compact bone mineral is higher in adults than in children it cannot yet be decided whether the increases seen in the treated patients are related to the sex hormone treatment, or reflect only the normal development of the bone during adolescence.