Androgen Reduction in Congenital Adrenal Hyperplasia, Phase 1

Inclusion Criteria: 1. Pre-pubescent girls (age 2 years \[12 kg minimum\] to 8 years inclusive; skeletal age \<10 years) or boys (age 2 years \[12 kg\] to 9 years inclusive; skeletal age \<11 years). 2. Confirmed classic 21-hydroxylase deficiency evident by genotype groups A, A1 or B or clinical course (e.g., adrenal crisis with documented hyperkalemia and hyponatremia, at diagnosis or during a later evaluation; ambiguous genitalia in females). Documentation of one or both parents' genotypes may be required to confirm the subject's genotype. 3. Requirement for standard of care fludrocortisone (any dose) and ≥10 mg/m2/day of hydrocortisone for at least 1 month prior to the study consent. 4. Morning serum androstenedione concentrations \>1.5 x Upper limit normal (ULN) after 7 days of dosing with doses of hydrocortisone required for physiologic replacement. 5. At least one parent (or other legally acceptable representative) must sign the informed consent form before the performance of any study procedures, but both parents must sign if both have parental rights. Children who are capable of providing assent (typically 10 years of age and older) must sign an assent form before the performance of any study procedures Exclusion Criteria: 1. Evidence of central puberty: Tanner Stage \>2 for breast development in girls or testicular volume \>4 mL in boys, or random luteinizing hormone (LH) \>0.3 milli-international units (mIU)/mL. Subjects with pubic and/or axillary hair as the only sign of puberty onset will be allowed. 2. Current or history of hepatitis from any etiology, including history of active viral hepatitis A, B, or C. 3. Patients with baseline hepatic impairment are excluded from this trial. To be eligible for this protocol, patients must meet all of the following criteria: AST, ALT and Total bilirubin \< ULN Albumin \> lower limits of normal (LLN) No evidence of ascites No evidence of encephalopathy 4. Abnormalities of liver function developing during the study 5. Abnormal renal function tests, defined as blood urea nitrogen (BUN) or creatinine \>1.5 ULN for age. 6. Significant anemia (hemoglobin \< 12 g/dl). If documented to be due to iron deficiency, subjects may be rescreened 3 months after this has been treated. 7. Clinically significant abnormality in the 12-lead electrocardiogram (ECG) 8. A history of a malabsorption syndrome. 9. Evidence of active malignancy. 10. Serious or uncontrolled co-existent disease, including active or uncontrolled infection. Subjects may be rescreened after resolution of any such condition. 11. Concurrent medical condition or disease other than 21-hydroxylase deficiency that may interfere with linear growth or that requires concomitant therapy that is likely to interfere with study procedures or results. 12. Asthma or other condition requiring treatment with systemic corticosteroids within the past 3 months. Asthma treatment with inhaled corticosteroids is permitted. 13. Treatment with potentially hepatotoxic medications (statins); strong inhibitors of CYP3A4 (ketoconazole, itraconazole, clarithromycin, atazanavir, nefazodone, saquinavir, telithromycin, ritonavir, indinavir, nelfinavir, voriconazole), or CYP3A4 inducers (e.g., phenytoin, carbamazepine, rifampin, rifabutin, rifapentine, phenobarbital). CYP2C8 substrates (rosiglitazone, pioglitazone, rapaglinide) and CYP2D6 substrates (dextromethorphan, thioridazine) should be avoided 14. Treatment with medications to affect puberty or synthesis of sex steroids, including gonadotropin releasing hormone agonists, aromatase inhibitors, or androgen receptor blockers (e.g., flutamide, spironolactone). However, a gonadotropin releasing hormone agonist may be started during the study for treatment-emergent central puberty without disqualifying the subject 15. Treatment with growth hormone at enrollment or during the course of the study. 16. Known allergies, hypersensitivity, or intolerance to abiraterone acetate or its excipients (refer to United States Prescribing Information). 17. Has received an investigational drug within 4 weeks of the planned first dose of study drug or is currently enrolled in an investigational interventional study. 18. Any condition that, in the opinion of the investigator, would make participation not be in the best interest (eg, compromise the well-being) of the subject or that could prevent, limit, or confound the protocol-specified assessments. 19. Presence or history of cataracts.