Context: Because many women with 21-hydroxylase (21-OH)-deficient nonclassic adrenal hyperplasia (NCAH) carry at least one allele affected by a severe mutation of CYP21, they are at risk for giving birth to infants with classic adrenal hyperplasia (CAH). Objective: Our objective was to determine the frequency of CAH and NCAH infants born to mothers with 21-OH-deficient NCAH. Design and Setting: We conducted an international multicenter retrospective/prospective study. Patients and Methods: The outcome of 203 pregnancies among 101 women with 21-OH-deficient NCAH was reviewed. The diagnosis of 21-OH-deficient NCAH was established by a basal or post-ACTH-stimulation 17-hydroxyprogesterone level of more than 10 ng/ml (30.3 nmol/liter). When possible, genotype analyses were performed to confirm CAH or NCAH in the offspring. Results: Of the 203 pregnancies, 138 (68%) occurred before the mother's diagnosis of NCAH and 65 (32%) after the diagnosis. Spontaneous miscarriages occurred in 35 of 138 pregnancies (25.4%) before the maternal diagnosis of NCAH, and in only four of 65 pregnancies (6.2%) after the diagnosis (P < 0.002). Four (2.5%; 95% confidence interval, 0.7-6.2%) of the 162 live births were diagnosed with CAH. To date, 24 (14.8%; 95% confidence interval, 9.0-20.6%) children, 13 girls and 11 boys, have been diagnosed with NCAH. The distribution of NCAH children and their mothers varied significantly by ethnicity (P < 0.0001, for both). Conclusions: The risk of a mother with 21-OH-deficient NCAH for giving birth to a child affected with CAH is 2.5%; at least 14.8% of children born to these mothers have NCAH. Copyright © 2006 by The Endocrine Society.