Cryptic 21-hydroxylase deficiency has been previously described in asymptomatic family members of patients with classical congenital adrenal hyperplasia (CAH). These family members were detected by high baseline 17-hydroxyprogesterone levels found in the course of family studies. The hormonal responses to ACTH of the family members with cryptic 21-hydroxylase deficiency were determined and compared to the responses of patients with CAH, patients with acquired adrenal hyperplasia, family members predicted to be heterozygous for CAH, family members predicted to be unaffected, and the general population. The ACTH-stimulated levels of 17-hydroxyprogesterone and Δ4-androstenedione in the cryptic family members were elevated above the level of the general population or family members heterozygous for classical CAH, but below that of patients with CAH. The hormonal profile of patients with cryptic 21-hydroxylase deficiency is similar to that of patients with acquired adrenal hyperplasia. The response of family members heterozygous for the cryptic gene (21-OH(CRYPTIC)/21-OH(NORMAL)) was indistinguishable from that of family members heterozygous for the classical CAH gene (21-OH(CAH)/21-OH(NORMAL)). These studies support our previous proposal that patients with cryptic 21-hydroxylase deficiency are genetic compounds, having one gene for a severe enzyme deficiency and one gene for a mild 21-hydroxylase deficiency. Thus, the 21-hydroxylase genotype in cryptic 21-hydroxylase deficiency is 21-OH(CAH)/21-OH(CRYPTIC).