This is a Phase 3 study to evaluate the efficacy, safety, and tolerability of crinecerfont versus placebo administered for 28 weeks in approximately 81 pediatric participants with classic congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency. The study consists of a 28-week double blind, placebo-controlled period, followed by 24 weeks of open-label treatment with crinecerfont. Subsequently, participants may elect to participate in the open-label extension (OLE) period. The duration of participation in the study is approximately 14 months for the core study and will be a variable amount of time per participant for the OLE (estimated to be approximately 3 years).

Inclusion Criteria:

• Be willing and able to adhere to the study procedures, including all requirements at the study center, and return for the follow-up visit.
• Have a medically confirmed diagnosis of classic CAH due to 21-hydroxylase deficiency.
• Be on a stable steroid regimen.
• Have elevated androgen levels.
• Participants of childbearing potential must be abstinent or agree to use appropriate birth control during the study.

Exclusion Criteria:

• Have a diagnosis of any of the other forms of classic CAH.
• Have a history of bilateral adrenalectomy, hypopituitarism, or other condition requiring chronic glucocorticoid therapy.
• Have a clinically significant unstable medical condition or chronic disease other than CAH.
• Have a history of cancer unless considered to be cured.
• Have a known history of clinically significant arrhythmia or abnormalities on electrocardiogram (ECG).
• Have a known hypersensitivity to any corticotropin-releasing hormone antagonist.
• Have received an investigational drug within 30 days before initial screening or plan to use an investigational drug (other than the study drug) during the study.
• Have current substance dependence or substance (drug) or alcohol abuse.
• Have had a significant blood loss or donated blood or blood products within 8 weeks prior to the study.