Congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency is an autosomal recessive condition in which deletions or mutations of the cytochrome P450 21-hydroxylase gene cause glucocorticoid and often mineralocorticoid deficiency. Despite optimal substitution therapy, control of classical CAH is often inadequate at puberty, and the problems encountered relate to hypocortisolism and/or hyperandrogenism. A number of physiological alterations in the endocrine milieu at puberty, which include alterations in the growth hormone/insulin-like growth factor axis, insulin sensitivity, as well as the activity of enzymes participating in cortisol metabolism and adrenal steroidogenesis, may account for the documented hypocortisolism and elevated androgen production, and may explain the difficulty in maintaining adequate adrenocortical suppression in pubertal patients with classical 21-hydroxylase deficiency.