OBJECTIVE Oestrogens are used to inhibit growth in girls with constitutionally tall stature. We studied the changes in different hormones that accompany such therapy. METHODS In this longitudinal study we examined the levels of total insulin-like growth factor-I (IGF-I), free thyroxine (FT(4)), thyrotrophin (TSH), testosterone, dehydroepiandrosterone sulphate (DHEA-S), cortisol and prolactin in two groups of girls receiving ethinyloestradiol at a dose of either 0.1mg daily (group A, n=22) or 0.2mg daily (group B, n=36). Hormonal measurements were performed at start of therapy and after 3, 6 and 12 months. RESULTS In both groups the levels of IGF-I, testosterone and DHEA-S were reduced while the concentrations of cortisol and prolactin were increased. The pituitary-thyroid axis was not significantly affected by this therapy. The girls receiving 0.2mg ethinyloestradiol daily had lower IGF-I levels after 12 months of therapy and had higher serum prolactin concentrations than the girls treated with 0.1mg daily. The reduction in predicted height and the advancement in bone age were similar in both groups. CONCLUSIONS Therapy with pharmacological doses of ethinyloestradiol changes the levels of several hormones including IGF-I, testosterone, DHEA-S, prolactin and cortisol but the role of the respective changes in the inhibition of growth is not clear. The suppression of DHEA-S levels by 40% suggests that the ovaries contribute significantly to the production of this hormone in pubertal girls.