Steroid 11β-hydroxylase deficiency is the second most common cause of congenital adrenal hyperplasia, the inherited inability to synthesize cortisol. Severely affected patients carry mutations in the CYB11B1 gene that destroy enzymatic activity. Such patients have signs of androgen excess and usually have hypertension. Mild or non-classic 11β-hydroxylase deficiency has been reported previously but not studied genetically. In this study we report analysis of the CYP11B1 genes of three patients thought to suffer from non-classic 11β-hydroxylase deficiency. Mutations were detected in the CYP11B1 genes of two patients. One was a compound heterozygote for missense mutations N133H and T319M, whereas the other carried a nonsense mutation (Y423X) on one allele and a missense mutation (P42S) on the other. All three missense mutations affected enzymatic activity when expressed in vitro. No mutations were detected in the coding regions or intron-exon boundaries of the CYP11B1 genes of the other putative non-classic patient. In addition, we were unable to detect CYP11B1 mutations in two hirsute women with mildly elevated levels of 11β-hydroxylase precursors who had previously been identified in a screening study of patients in a reproductive endocrinology clinic. We conclude that non-classic 11β-hydroxylase deficiency is a rare disorder. It is not a significant cause of hyperandrogenism in women and relatively stringent criteria should be used to prevent its misdiagnosis.