21-hydroxylase (21-OH) deficiency accounts for the vast majority of nonclassic (NC) forms of congenital adrenal hyperplasia (CAH), and is associated with symptoms detectable either in childhood (precocious puberty) or sometimes only later in adulthood (hirsutism, acne, amenorrhea). While the severe forms of the disease responsible for salt wasting or simple virilization have been extensively studied, the NC 21-OH deficiency is less well characterized, especially in adults. We studied the 21-OH gene (CYP21) in a population of 69 unrelated hyperandrogenic subjects suspected to be homozygous or heterozygous for NC 21-OH deficiency, based on basal and adrenocorticotrophin (ACTH)-stimulated plasma 17-hydroxyprogesterone (17-OHP, 17-OHPSI) and 21-desoxycortisol (21-DOF, 21-DOFSI) levels. To identify all mutations involved, determination of the whole gene sequence, including exons, exon-intron junctions, and promoter region, was performed, followed by a study of large rearrangements and identification of compound heterozygotes. Alterations were identified in at least one allele of 55 hyperandrogenic subjects. Two NC alterations, Val282Leu and Pro454Ser, were detected in 68% and 7% of the affected alleles, respectively, whereas mutations involved in severe forms were identified in 21% of them. These results document the utility of a molecular diagnosis in hyperandrogenic women suspected of being either heterozygous or homozygous for NC 21-OH deficiency and clearly indicate the importance of genetic counseling in such a population.