This study is a parallel arm, randomised, open-label study, including dose titration and admissions for four overnight stays for 24-hour endocrine profiles. It will compare the efficacy, safety and tolerability of Chronocort® with standard glucocorticoid replacement therapy in the treatment of congenital adrenal hyperplasia (CAH) over a treatment period of 6 months. Dose titration decisions in both treatment groups will be made by a central independent physician, blinded to the treatment arm, using information generated from the 24-hour endocrine profiles. Each treatment arm will be subject to the same titration rules throughout the study, ensuring that opportunities for optimisation and control of androgens are the same in both groups.