Exogenous glucocorticoid replacement in patients with congenital adrenal hyperplasia (CAH), who due to an adrenal 21-hydroxylase enzyme deficiency are unable to produce endogenous glucocorticoids, is aimed at normalizing hypothalamic-pituitary-adrenal function. Excess androgen production by the adrenals is thus decreased. Despite standard glucocorticoid replacement doses (12.5-40 mg, 10.5-27 mg/m2/day hydrocortisone equivalents) 4 of 7 patients ranging in age from 14 to 33 years had abnormalities of the pituitary on MRI. Three appeared to have microadenomas and 1 had an empty sella. Five (3 salt wasters, 2 simple virilizers) of these 7 patients had 60-min p.m. ovine corticotropin-releasing hormone (oCRH) stimulation studies. The mean (logarithm) area under the ACTH curve for 0-60 min after oCRH stimulation was significantly greater in patients than controls (p < 0.0001). Mean ACTH at each time point before and after oCRH stimulation was similarly greater in patients than controls (p < 0.05). Two of these patients had pituitary microadenomas, 1 had an empty sella; all 3 were salt wasters. Despite standard glucocorticoid replacement, adolescent and young adult patients with CAH tend to have high basal ACTH and ACTH hyperresponsiveness to oCRH, as well as structural abnormalities of the pituitary. The inevitable periods of under- and overexposure to glucocorticoids in CAH patients may over time cause abnormalities of the hypothalamic-pituitary-adrenal axis.