Inclusion Criteria:

• Participant must be aged 16 years or older at the time of signing the informed consent/assent.
• In participants aged < 18 years, height velocity must be less than 2 cm/year in the last year and puberty must be completed (Tanner stage V).
• Participants with known classic CAH due to 21-hydroxylase deficiency diagnosed in childhood with documented (at any time) elevated 17-OHP and with or without elevated A4 and currently treated with hydrocortisone, prednisone, prednisolone or dexamethasone (or a combination of the aforementioned glucocorticoids) and on stable glucocorticoid therapy for a minimum of 3 months.
• Participants who are receiving fludrocortisone must be on a documented stable dose for a minimum of 3 months prior to enrollment and must have stable renin levels at baseline.
• Male and female participants.
• Female participants of childbearing potential and all male participants must agree to the use of an accepted method of contraception during the study.
• A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies:
• Not a woman of childbearing potential (WOCBP); OR
• A WOCBP with a negative pregnancy test at entry into the study.
• Capable of giving signed informed consent/assent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol.

Exclusion Criteria:

• Clinical or biochemical evidence of hepatic or renal disease; e.g., creatinine > 2 times the upper limit of normal (ULN) or elevated liver function tests (alanine aminotransferase [ALT] or aspartate aminotransferase [AST] > 2 times the ULN).
• History of bilateral adrenalectomy.
• History of malignancy (other than basal cell carcinoma successfully treated > 26 weeks prior to entry into the study).
• Participants who have type 1 diabetes or receive regular insulin.
• Persistent signs of adrenal insufficiency or the participant does not tolerate treatment at the end of the 4-week run-in period.
• Participants with any other significant medical or psychiatric conditions that in the opinion of the Investigator would preclude participation in the study.
• Participants on regular daily inhaled, topical, nasal or oral steroids for any indication other than CAH.
• Co-morbid condition requiring daily administration of a medication or consumption of any material that interferes with the metabolism of glucocorticoids (examples provided at http://medicine.iupui.edu/clinpharm/ddis/clinical-table/).
• Participants who are receiving < 10 mg hydrocortisone dose at baseline or the hydrocortisone dose equivalent.
• Participants anticipating regular prophylactic use of additional steroids; e.g., for strenuous exercise.
• Participation in another clinical study of an investigational or licensed drug or device within the 12 weeks prior to screening.
• Inclusion in any natural history or translational research study that would require evaluation of androgen levels during the study period outside of this protocol's assessments.
• Participants who have previously been exposed to Chronocort in any Diurnal study.
• Participants who routinely work night shifts and so do not sleep during the usual night-time hours.
• Participants, who in the opinion of the Investigator, will be unable to comply with the requirements of the protocol.

Eligibility last updated 2/17/22. Questions regarding updates should be directed to the study team contact.