This study will test a new, extended release form of hydrocortisone called Chronocort in patients with congenital adrenal hyperplasia (CAH). People with CAH do not make enough of the adrenal hormones cortisol and aldosterone, and their adrenal glands make too much of the sex hormone androgen. Medicines called glucocorticoids (hydrocortisone, dexamethasone and prednisone) are currently used to treat CAH, but finding the best dose of these drugs that effectively lowers androgens without causing undesirable side effects, such as weight gain and slow growth rate in children, is often difficult to achieve.