PurposeNonclassic 21-hydroxylase deficiency, a mild form of congenital adrenal hyperplasia (CAH), is estimated to be the most common autosomal recessive condition, with an especially high prevalence in Ashkenazi Jews (3.7% affected, 30.9% carriers), based on a 1985 HLA-B linkage study of affected families. Affected individuals, especially women, may suffer from hyperandrogenism and infertility. State-of-the-art genetic studies have not been done to confirm these remarkable rates.MethodsCYP21A2 genotyping was performed in 200 unrelated healthy Ashkenazi Jewish subjects and 200 random US Caucasians who did not self-identify as a specific ethnicity using multiplex minisequencing, real-time polymerase chain reaction and junction site analysis.ResultsNonclassic CAH carriership was found similarly in 15% (95% confidence interval (CI): 10.4-20.7) of Ashkenazi Jews and 9.5% (95% CI: 5.8-14.4) of Caucasians (P=0.13). The proportion of Ashkenazi Jewish nonclassic CAH carriers (0.15 versus 0.309, P<0.0001) and disease affected (0.005 versus 0.037, P=0.009) was not as high as previously reported. The estimated prevalence of nonclassic CAH in Caucasians was 1 in 200 (0.5%, 95% CI: 0.01-2.8).ConclusionNonclassic CAH is a common condition, regardless of ethnicity, and should be considered with preconception and infertility counseling.