Adolescent females who have irregular menstrual periods may have the nonclassical form of congenital adrenal hyperplasia due to a mild deficiency of steroid 21-hydroxylase (NC 21-OHD). Hyperandrogenic signs such as acne, frontal hair loss, hirsutism, and irregular menstrual periods should alert the physician to the diagnosis of NC 21-OHD. An ACTH stimulation test in which serum hormone concentrations of 17-OHP, Delta(4)-androstenedione, and testosterone are determined will assist in the diagnosis of NC 21-OHD, but the definitive diagnostic test is an analysis of the mutations in the CYP21A2 gene. Typical mutations in the CYP21A2 gene in patients with NC 21-OHD are an exon 7 or an exon 1 mutation. Once the genotype establishes the diagnosis of NC 21-OHD, treatment should be initiated. Typical treatment is dexamethasone, 0.25 mg HS, which generally reverses the hyperandrogenic signs.